It says here that it's roughly 10% of machines. It's safe to assume at least half are infected. But aren't they mostly third world?
https://www.netmarketshare.com/operating-system-market-share...
https://encrypted.google.com/search?hl=en&q=android%20botnet...
https://encrypted.google.com/search?hl=en&q=android%20botnet...