IMO its not specifically RP servers that have huge number of players. Its game modes with a defined objective that have more players (normally). (RP's have loosely defined objectives, depending on which character you choose)
There are a lot of TDM/"call of duty"/cop and robbers/PTP/race+stunt games modes which have more players. Also, time zones also make a difference. ATM "Black ops" have 30 players, when i searched at 7pm 2 days ago they had 100+.
(Why stunt servers get so many I don't know, unless there are objectives but there's always an odd exception to the rule - Like party server is as well lol)
Well there's my two cents for the week.