As your business needs grow, you can start layering complexity on top. The point is you don't start at 11 with a overly complex architecture.
In your example, if your server crashes, just make sure you have some sort of automatic restart. In practice that may mean a downtime of seconds for your 12 users. Is that more complexity? Sure - but not much. If you need to take your service down for maintenance, you notify your 12 users and schedule it for 2am ... etc.
Later you could create a secondary cluster and stick a load-balancer in-front. You could also add a secondary replicated PostgreSQL instance. So the monolith/postgres architecture can actually take you far as your business grows.