It's not a stupid question, Charles was the one probably
UBB is written well, but your number of members and posts can slow it down significantly if your server isn't setup correctly - throw enough hardware at it and you can just about run any size forum.
In other words, hardware is the limiting factor more than anything. Most people run on shared servers, >30k members would significantly slow your forums down. Put your site on a fast server with scsi disks, xeon processors, and loads of ram and you can run it way higher than that. I've seen ubb's with 600+ people on at the same time, it mostly depends on hardware
