What is the max number of database I can create in an instance of pgsql?

Hi,

We want to create a database for each one of our departments, but we only
want to have one instance of postgresql running.  There are about 10-20
departments.  I can easily use createdb to create these databases.  However,
what is the max number of database I can create before performance goes
down?

Assuming each database is performing well alone, how would putting 10-20 of
them together in one instance affect postgres?

In terms of getting a new server for this project, how do I gauge how
powerful of a server should I get?

Thanks.