How PostgreSQL handles multiple DDBB instances?

Hi all,

I have a doubt/problem about how PostgreSQL handles multiple DDBB
instances running on a same server and how I should design the
architecture of an application.

I have an application that works with multiple customers. Thinking in
scalability we are thinking in applying the following approaches:

- Create a separate database instance for each customer.
- We think that customer's DB will be quite small, about 200MB as
average.
- The number of clients, then DDBB, can be significant(thousands).
- Have as many customers as possible on the same server, so a single
server could have more than 300 DDBB instances.

Do you think this makes sense? or taking into account that the
expected DDBB size, would be better to join several customers DDBB in
just one instance. What I'm worried about is, if having so many DDBB
instances PostgreSQL's performance would be worse.

I have been following the list and one of the advises that appears
more often is keep your DB in memory, so if I have just one instance
instead of "hundreds" the performance will be better?

Thank you very much
--
Arnau