I cannot find a definitive answer on this. I am dealing with a rapidly growing WordPress MU install (horrible DB design) running on MySQL 5 and RHEL 5. How many tables can fit in a single DB before there will be some issues?
It has more to do with the underlying OS/filesystem than it does with MySQL - hence why you can't find an answer
If you need a number (Linux): ~50 000.
You won't get that number on a small ec2 instance, but if you've got a little bit of memory it should work. You will need to raise the table_cache quite high, and you'll sink a lot of lost memory lost in InnoDB data dictionary.