You can use the one postgresql instance and have multiple databases on that instance. You do not need to create separate instances (one for each database). If you have specific requirements because maybe a queue database or cache database has different infrastructure requirements, then you should be able to set up additional servers and use the accessories for each of those servers to host their own container of postgresql. But if we're talking a more simple setup (everything on one box/vm) then you would need just the one postgresql instance (even with multiple databases).