About that 1800 players number. Based on posts by one of the developers, their server doesn't do any sort of calculation:
> Our server is basically 'dumb', it doesn't do anything in the sorts of processing bullet trajectory, player line of sight or any hit detection at all.
http://www.jc-mp.com/forums/index.php/topic,638.msg4783.html...
Unlike other games which run a full simulation on the server-side, their server (I presume) mostly routes messages between players. This means that the server trusts the client, which is rather poor design for a competitive multiplayer game. Once this is released, players will probably prefer a cheat-free game to a large-scale one.
This is, nevertheless, a fantastic mod. But these people have not yet solved the c10k-equivalent problem for massively multiplayer games.