Do you have any experience about this question? I have currently 1900 MySQL databases in a single domain in my plesk control panel and I wonder if my MySQL server gets overloaded or out-of-service due to such high number of databases in the system.
Do you have any suggestions? Each database is for a user in my service by the way.
MySQL itself doesn't place any restrictions on the number of databases you can have, and I doubt Plesk does either, I'm sure it just displays all the databases present on the MySQL server.
However, your host may have a limit (which you'd have to ask them about), or if you start getting a huge number of databases, you may actually run into a filesystem limit. As the MySQL documentation says, each database is stored as a directory, so you could hypothetically hit the filesystem's upper limit for how many subdirectories are allowed.
ive got well over 5000 databases running on alinux based plesk cluster (one db one web server) and its running fine, though i have had to increase open files limits due to the huge amounts of files. i cant run the mysql tuning primer any more though, well i can but it takes about 4 hours