No more than 16. Sure the game will technically support it, but just because you can doesn't mean it's a good idea.
Just curious, why isn't it a good idea? I've seen a couple servers online with 32 players max, are these unstable or otherwise problematic? Or is it just a gameplay thing?
p.s. to the OP, please put the topic of your question in the Subject box when posting, it makes it easier for people browsing the board to know what the thread is about. Thanks.