Maybe the Jedi Outcast community is bigger than Jedi Academy idk (I heard people still play) or what if there is some other Jedi Academy mod with a community as big as MBII?
From what I know, while it may not have the most players total that get on occasionally, the mb2 community seems to be the most active. other mods such as force mod, or ja+ are packed with servers, but they just have a bunch of bots and it's very off putting for the average gamer
Population depends on your time zone but generally swings from 30-80 active players as of late (not bad considering launcher / serverlist were broken for over a month). Player numbers were peaking over 100 before the disaster, so I expect things to recover to that within the next few weeks.
As to OP, I would say this is the single largest community focused on something homogeneous. jkhub.org is a kind of conglomeration of various groups from those who are left outside the MBII community. There are probably still a few of the larger base clan groups still going too, though I'd bet their numbers have dwindled by this point.