Currently the actual number is assumed to be around 800,000-1,000,000. I'd say not more that low 1 millions. Maybe 1.5 but only in rare instances.The are some sources stating that Bagdad at it's height had almost 2 million, so I thought a slightly larger caliphate centered there would be able to push that a little further. (Or maybe a Roman Constantinople that fends off the Arabs, or a Tang Xian that manages to hold Central Asia and Korea, and possibly also a larger Angkor Wat could pull off such numbers)
And let me ask my perennial question, how was sanitation handled in the Muslim cities? You have discussion of China, India, Europe, Africa, but the Middle East is skipped. I've been trying to find the answer for some time. But all you get is stuff about personal hygiene.