There's the question of isolation. One shared service going down brings down multiple applications. But it can be more resource efficient to have a single DB running. If you're doing database maintenance it can also be easier to have one instance to tweak. But if you break something it impacts more things...
Generally speaking I lean towards one db per application. Often in separate containers. Isolation is more important for what I want.
I don't think anyone would say you're wrong for using a single shared instance though. It's a more "traditional" way of treating databases actually (from when hardware was more expensive).