It doesn't - Edit 1
Before modification by Floffe at 13/05/2011 10:29:52 AM
Some people wouldn't visit some boards at gunpoint, three "registered" users (that I know) have never posted and never will, and others may have registered without posting (particularly in the brief interval when RAFO and wotmania were both online) so I'd estimate 725±25 users. I may be overlooking some huge factor(s), but I'd be very surprised if it were >800.
You're not off by an order of magnitude, but about half that: a factor of circa 3.
Seeing as every user has a number, which can be seen by going to their profile, I decided to find the lowest and highest number. The lowest is 100, that being Ben, and the highest is 2012. That gives 1913 registered usernames.
Of course, some of these will be secondary accounts, some will have left and so on, but they are still registered.