A lot. In 2019, the most recent estimates put the number of app developers at 26.4 million globally, of which around 6 develop solely for Android, and 2.8 million focus on iOS.
In 2014 , Asia held the highest number of app developers with 760,000 (32.9% of total app developers at the time), swiftly followed by Europe and North America with 29.7% and 29.4% of devs. Keep in mind however that since then around 800,000 new developers have entered the market per year.