No limit. Do they show in the control panel and not just the main forum listing? If so, then clear out the cache. You might need to manually do it by ftp'ing into your site and deleting the cache/forum_cache.php file.
This was fixed via a ticket. When looking at the forums that didn't display, they didn't have any permissions set. I just used the Copy option from one of the similar forums permissions, and all was fixed.