[kmike]

The main problem isn't in the datastore size, but in that the whole list of forums is re-processed by many of the main forum scripts (forumdisplay.php, index.php and basically everywhere if forumjump is enabled). I think you can go up to a few thousands forums without any noticable impact if you have a powerful web frontend. Or you can cache the processed forum list (iforumcache) for some time but not for long as it contains some important info.

I can't see how you can split the forums across several servers if you want all of them displayed in one list.