Hard to say how many players. But, there is a pretty consistent group that play on the Mystic-Fighters server on Monday nights (German time). And, there is a consistent group that play on the BBF server on Monday, Friday, and Saturday nights (USA time). It is hard to predict where the action is going to be happening at any given day or time.
I suggest getting xFire, and then setting your Unreal Tournament 2004 server filter to look for "xmp" map names. When you see some action starting up, hop in. We love to have new players and will help you along to learn the game if you need it. Don't be afraid to ask for help/explanation in-game.