I dont know exactly, it is a fairly extreme unique game. Its an fps with rts elements I'd say and very irritating to play at times (ie. learning it) so most wont like it. You have to stare at a black screen for 30 seconds every time you die, if you really screw up it could be as much as 5 minutes thats vs 15 seconds I think bf2 has.
I would guess no more then 10,000 players, so tiny compared to bf2 but it does have a strong regular player base including members of the british army who play as ABA - actual british army and its the only game to have a server paid for by the british army much like that americas army game
The 9th most populated bf2 server in the world is one running PR and its the most popular mod for bf2
http://www.game-monitor.com/search.php?game=bf2&vars=gamevariant=pr&public=&num=100&order=grank