Upvote:-1
I couldn't leave a comment because my reputation is too low, however, I will do some rough calculation for you. Note, it is probably impossible to get a good estimate due to a range of reasons (plagues, extent of borders, lack of data available).
The worlds population in 700 CE was estimated to be between 207 Million and 220 Million. In 800 CE, the worlds population was estimated to be somewhere between 220 Million and 240 Million.
Let's say the world population in 750 is 220 Million, and that the Umayyad Caliphate held roughly 29% of that population at its greatest height, according to this article. 29% of 220 Million is 63.8 Million. This dramatic rise could be due to territory expansion.