Try 7405, at time of writing: http://archive.gamespy.com/stats/
That's an impressive figure!
Plus, with UT2004 you have to take into account that most people who own it and play it regularly play offline with the fantastic Bots. No need to find servers and deal with lag when the bots behave just like humans - humans that actually know what they're doing...and who don't bitch you out for lagging their server with you're 56k modem :). I play Instagib matches against Godlike Bots daily, and it doesn't get old.