Maximum number of databases for single instance of PostgreSQL 9
I haven't tried it myself, but there are others around who have.
Here you can see that even 10,000 databases run without problem on a single instance.  You can even find some pratical aspects on ServerFault as well.

Since your databases are quite small, you will not run into any sort of file number limitation of the host OS.  The only problem I can think of is that when all these databases will be accessed concurrently, handling all those connections will be tricky.

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