How many people believe in Allah?
All Muslims, Christians, and Jews believe in the same God called in Arabic Allah. However, if you mean how many Muslims, then the answer is 1600 Million Muslims. see link below.
Only Muslims believe in Allah and about 1.5 Billion people believe in Allah. Eventhough Allah means God in Arabic, the god Allah is different from the God of Jews and Christians. Allah was worshipped as the main Moon god in Kaaba in pre-Isalmic times.
Chrisians believe Jesus is God aswell
Muslims believe that Allah is One and Only. That Allah has no parents, offsprings or partners in power. They believe that Muhammad was a human and an apostle of Allah. They believe that Quran is the last and the most authentic book of Allah. They also believe that they will be resurrected after death.