For desktop, probably only one, but who uses "desktop computers" any more?
Now that your phone, your music player, your computer, your notebook, your tablet, your game console(s), your television and your video playback device all need an OS, I'm guessing the number of operating systems people use on a daily basis is higher than you think.
Now that your phone, your music player, your computer, your notebook, your tablet, your game console(s), your television and your video playback device all need an OS, I'm guessing the number of operating systems people use on a daily basis is higher than you think.