Also, I have read there is an estimated 1.2 billion muslims in the world, but that some people think this figure to be inaccurate as many of these muslims do not practice being a Muslim and choose to be ignorant. Are they still considered Muslims or not?
Islam and Iman or faith (which are two terms referring to two different things) have different levels and degrees. The first degree, which is referred to as “Islam”, can be reached when one says the shahadatain (the two statements of “There is no God but Allah” and “Muhammad is the messenger of Allah), admitting to its meaning, resulting in that person officially becoming Muslim. His/her body will be pure (no longer najis) and so will the bodies of his children, marriage with other Muslims will become permissible for him and he/she can have transactions with other Muslims, his/her wealth, life and honor will all be respected etc. Of course, saying the shahadatain and admitting to its meaning necessitates acting and practicing Islam (e.g. performing the wajib acts such as prayer, fasting, Hajj, Khums and Zakat, and believing in the angels, the hereafter, paradise and Hell and accepting all of the divine prophets as messengers of Allah (swt)), which is the next level of faith which is higher than just plain “Islam” and being Muslim.
In addition to performing the mandatory acts, restraining from the haram and prohibited ones also causes one’s level and degree to rise even more. Considering the affirmative commands of the Quran and the Prophet’s and imams’ instructions and recommendations, all of the above (saying and admitting to the meaning of the shahadatain, practicing Islam and abiding by all of its affirmative and negative commands) still won't take their bearer to the highest level of faith and perfection unless that person accepts the religious authority (which is an absolute authority in all fields) of the twelve infallible imams. One’s faith won't be accepted by Allah (swt) without it. Clearly, a true believer must be a true believer on the inside as well and not a hypocrite, because in that case one’s good acts (such as prayer, fasting etc.) won't do him/her any good, and won't cause him/her to perfect and reach eternal prosperity. The only thing that good acts will do for a person who truly isn’t a believer on the inside, is to buy him/her Allah’s (swt) anger.
Therefore, all of the “Muslims” in the world today (1.2 billion) who have admitted to the shahadatain might be Muslim, but are at the lowest level of faith. Yet, if they don’t practice Islam, that doesn’t mean that they aren’t Muslim anymore.
“Islam” literally means submission and obedience. This term refers to the religion brought by the Holy Prophet Muhammad (pbuh) from Allah (swt) as the final divine religion to come, which is a universal and eternal one (meaning that no other religion will come after it that abrogates it until the day of judgement).
The most important thing that sets a difference between this religion and other ones, is the belief of the oneness of the great Lord, the Prophet Muhammad (pbuh) being the seal of all prophets, and its teachings on true tawhid. Believing in Islam and having faith in it, has different degrees which are as follows:
The first degree and step in Islam, which is the necessary condition to being Muslim, is to admit to the two principles of tawhid (the oneness of Allah (swt)) and nubuwwah (prophethood). The concept of لا اله الا الله (there is no God but Allah (swt)), is the true essence of Islam and its most important doctrine, containing all of the different dimensions of tawhid in it, and admitting to the prophethood of Prophet Muhammad (pbuh), is the acknowledgement that he is the seal of the prophets and that his religion is the final one to come, nullifying all other religions before it (meaning that these religions can no longer be followed, although they might have been the religions to be followed before the advent of Islam). The rest of the degrees of higher faith start from here, which is to follow the Prophet Muhammad (pbuh) in all of his teachings and decrees without any questioning.
One who bears witness to the two principles mentioned above, has embraced Islam and parted from all other religions. When such takes place, all rulings pertaining to Muslims (such as the permissibility of marrying other Muslims, having transactions with them, the taharah (being pure and not najis) of their bodies and their children’s bodies, will all pertain to him/her as well  and his/her life and possessions will be respected and defending him/her will become a responsibility of the Islamic governor and the Muslim society. As was said, this is the least that every Muslim must bear and others can no longer call such Muslims Mushriks or Kafirs anymore, even though their faith is the lowest one can have, but then again, it is faith. Therefore, what some sects such as the khawarij have done by considering Muslims who don’t completely practice their faith and commit great sins as Kafirs, not respecting their lives and blood and allowing their killing, or what the Mu’tazilah have done by not considering the abovementioned neither kafir nor Muslim, or what the Wahhabis have done by announcing those who prostrate on the turbah during prayer or kiss the holy shrines of the imams or prophet (pbuh) mushriks (polytheists), or those who call anyone who differs with them in their beliefs mushriks, are all wrong and in misguidance!
In addition to what was said above, according to the ithna’ashari sect, considering the different verses and hadiths that imply this matter, accepting all twelve individuals as imams, leaders and successors to the holy Prophet (pbuh) after him is one the most important conditions of faith and the acceptance of our actions and worship from Allah (swt), because accepting the holy Prophet (pbuh) and the Quran as the final revelation to the prophet (pbuh) which hasn’t been changed and falsified till today, necessitates the precise and complete practice of the teachings of the Quran and the prophet (pbuh). One of these teachings and guidelines which is of extreme importance, is for us to obey the imams. Therefore, in reality, disobeying them is equal to disobeying the Prophet (pbuh) and Allah (swt) because they are the ones who have ordered us to follow them. In Quranic terms, the degree of iman comes after the degree of Islam. Thus, the first degree of iman is higher than the degree of Islam. Iman also has different degrees and all degrees of iman aren’t the same. The holy Quran says that the least that is needed to obtain the degree of iman is for one to have faith and belief in five things; 1- Allah (swt) 2- the day of judgement 3- the angels 4- the book (the Quran) 5- the prophets.  The Quran also counts hypocracy and shirk (polytheism) as turning away from Islam and the causes of eternal chastisement in the Hellfire  . Therefore, one is Muslim who:
1- Has accepted tawhid in all of its dimensions.
2- Believes in the prophethood of Prophet Muhammad (pbuh) and him being the seal of the prophets.
3- Accepts all of his commands, including the command to follow the imams and accept their authority.
4- Believes in the hereafter and its details as the Quran, the prophet (pbuh) and the imams have explained (of course, those details that we are sure the Prophet (pbuh) and the imams have described).
Nevertheless, although admitting to all of these things cause one to be considered Muslim, as long as Islam isn’t truly practiced through abiding and acting according to its rulings and teachings, such an Islam won't do any good to its possessor and will not bring him/her any guidance or prosperity in the hereafter. That is why the Quran says that reaching the prosperous life in the hereafter depends on faith and good acts together. 
One who only has one of the two resembles a bird that possesses one wing while lacking the other. Clearly, such a bird isn’t capable of flying to high altitudes, and the same goes with such a person; he/she will never be able to reach the high peaks of eternal prosperity through good deeds only or faith only. The only solution for such an individual is to change his/her course of action and to combine his/her faith and acceptance of Islamic teachings with good actions in order to get nearer to Allah (swt) and abide in His heaven. As the believer’s knowledge of Islam, faith, and the teachings of Allah (swt) and His prophet increases, and as his/her practice in relation to what Islam has asked him also increases and gets purer, his degree of faith will also rise, taking him to higher levels of faith.
Here, a few points need to made: 1- There is a two-way relationship between faith and righteous deeds, meaning that the more one’s faith grows, the more the quality and quantity of his/her actions will rise and the more he/she will stay away from the disobedience of Allah (swt). On the other hand, the more one’s actions perfect, and the more he/she stays away from sins, especially the great sins, the deeper the roots of faith grow in his/her heart, to the extent that the believer reaches his/her true salvation and the peaks of true humanity and perfection. The same goes for those who sin and exceed their limits in sinning, their faith will slowly begin to deteriorate and vanish from the hearts of their possessors. On the other hand, those who have a weaker faith are the ones who commit sins.
2- Accepting the other prophets of Allah (swt) and their books (not the books that have been changed throughout history, yet are related to the prophets), doesn’t necessarily mean that one has to practice their religions, because some of those religions were exclusively for a certain people, while others of them were nullified and invalidated by the coming of new religions (meaning that they were no longer to be followed, not that they were never to be followed). In other words, many of these religions’ expiration date has arrived and they have expired and can no longer be followed. Therefore accepting them means to respect them and their high ranks and their efforts, and recognize them as Allah’s (swt) messengers, not to follow their religions.
3- The most important forms of worship that the Muslim individual needs to practice, which are the border between one being Muslim or not, are referred to as “the branches of Islam”, and learning and practicing them is necessary for all Muslims.
Nevertheless, one who doesn’t practice them, without rejecting them as being the fundamentals of their religion, doesn’t make that person no longer Muslim, even though as was said before, it does cause one to slowly lose faith, and if not made up, causes the Muslim to be deprived of eternal prosperity and later punishment.
4- Faith needs to be whole and cannot be divided, meaning that if one is truly Muslim, he/she can't say: “I only believe in such and such part of Islamic teachings and don’t believe in the rest, because according to the Quran, such an account is merely the following of one’s desires and is considered kufr, not the belief in Allah (swt) and His prophet and the day of judgement. 
5- Both faith and righteous deeds have different degrees and can get weak or strong. All righteous believers aren’t the same when it comes to their degree of faith. Therefore, it is important to strive and work on both the quality and quantity of our good acts and worship in order to gain the highest degrees. 
6- Denying any of the principle beliefs of Islam (e.g. tawhid, prophethood,) and any of the branches of Islam (e.g. prayer, zakat) are all considered the denying of one of the fundamentals of Islam and in certain cases, cause one to longer be Muslim and be an apostate and of those who have turned away from Islam.
Conclusion: There is a difference between Islam (being Muslim) and Iman (faith), and and each have different levels and degrees. All of the 1.2 billion Muslims out there who have admitted to the shahadatain are Muslim and all Islamic decrees pertain to them just like every other Muslim, therefore not putting Islam to practice doesn’t cause one not to be Muslim anymore, although it won't do the Muslim individual much good either.
For further information, see indexes:
2- The contradiction between the Muslims’ acts and their beliefs, Question 659 (Website: ).
 Of course, there is a difference of opinion regarding the purity of the bodies of the people of the book, which you can learn about by referring to the different risalahs of various marjas.
 Baqarah:177 and 285; Nisa:136.
 Nisa:140 and 145.
 Nahl:97; Baqarah:103; Nisa:57 and 122.
 Baqarah:85; Nisa:150-151.
 With help from Question 148 (Website: ) (The most basic beliefs of Muslims).