It also depends on your OS that you;re running. I know we had a CPanel box way back when that had a limit of 12k files per a directory. While only 10 database tables are created per blog, three files are created for each of those tables or a total of 30 files per blog. Please note that I only saw that once, a normal limit is about about 30k files, and that host didn;t last long anyway.
Using the 12k file limit as an example, that would be a total of 400 blogs. Again, though, that was an artificial limit placed by a host.
We've discussed work arounds previously. The three main solutions are:
- Keeping up on spam and unused blogs. That'll only get you so far.
- Automattic's solution, Hyperdb, which is normally for rollover dbs but is also supposed to work with wpmu: http://wordpress.org/extend/plugins/hyperdb/ Trent uses this one I believe and I'm sure he'll chip in if he's around.
- Andrew and James' Multidb code with their premium site: http://premium.wpmudev.org/project/multi-db
Solutions 2 and 3 break down the database tables into collections of databases. Since the file restriction count is only on files within a directory, and each database creates their own directory within the mysql storage directory, this gets around most limits.
Hope this helps