Thanks, the stats page http://stats.cyanogenmod.com/ even shows 2.4 million, including unofficial installs. One has to add sth. to the number, if some users do not participate in the statistics.
I have no idea about the number of Debian users, less so if one counts in derivates, such as Ubuntu, Mint, etc. I would believe anything between 1,000 users (roughly the number of Debian developers) and 1,000,000,000 (roughly number of PCs in use worldwide).