How many database servers do I need? This may be a no-brainer to some of you, but I am a newcomer to DBA and this question popped into my head and I couldn't answer it on my own and so I am turning to the experts here.

I have a app that requires two databases –– one production and one for staging. Do I need 2 database servers, or should I use one server and simply keep them in different databases? 

On the one hand, you can create a new server using Docker containers so easily. I've been wondering if keeping the data for two environments on the same database is a safe approach.
You need at least two. Don't mess around in production.