[Marco van Herwaarden]

Even though vBulletin supports "unlimited" forums, in real life more then 1.500-2.000 forums will put a too high pressure on the server (performance and memory). The real maximum depends on the server and the server configuration. As vBulletin 3.x will cache each forum in the datastore (and will load this information into memory on each page load), you would quickly run out of memory.

Some boards that use this many (sub)forums are still using vBulletin 2.x only because that version does not do forum caching and will preserve memory in cases like this. One of the things that will be focussed on when developing vBulletin 4 (there is no indication on a date for that) is the reduction of memory usage for large boards.