First of all, every database comes with some metadata overhead (catalog tables) which amounts to ~ 30MB. Take into account that this also may (should!) be in me

First of all, every database comes with some metadata overhead (catalog tables) which amounts to ~ 30MB. Take into account that this also may (should!) be in memory (shared_buffers) so it can become expensive. But the most significant factor is whether you need to do queries that span multiple users/tenants. If you need to, you will require postgres_fdw and nor performance neither manageability will be good at all.