I'm not sure about how many users each server holds, though I assume it would be equal amounts; theres two ad free servers, which probably hold 50% each of all ad free accounts, and similar for the ad enhanced servers; though I may be wrong.
I remember reading somewhere that there is around 300 - 400 new registrations everyday; which is amazing in it's self. Take a look at the forum's statistics on the board index; there's over 52,694 members.