In general, if you have an active community with 20k members or more, the ubb is beginning to reach its limits without some really nice hardware backing it up. They encourage you to move up to threads or open topic around that point.
With nice dedicated hardware tho, you can pretty much go as high as you want as long as you keep throwing more memory and hard drive speed at it - true of most of the better forums.
The key is not so much how many you have registered, but the number online and posting at one time. I've seen UBB's with >600 online at once and doing fine. Of course, it was on a dedicated machine with dual processors and 1gb of ram.
