It's been mentioned in Steve's keynotes a few times.
I can't give you exact numbers, but there's roughly 25 million Mac users as the installed user base. Of this, I'd say Apple's somewhere in the 5-10 million range for OS X users.
Remember, the 25 million number includes Mac Plus users, etc. Basically, if they still use the Mac, it's counted. Obviously, these people can't upgrade to OS X, but they can (and most likely will) buy new Macs.