In Munich (as in many other German cities) there are two different systems: S-Bahn, managed by DB (the national railway operator), and U-Bahn, that is your standard subway, managed by a local company (MVG in Munich).

The S-Bahn in Munich has only one track in the city center, so there are trains every 3-4 minutes, 20 minutes is on the outskirt.

U-Bahn has a 10-minute cadence in the outskirt, and 5-minute on rush hours: in the city center 6 lines become 3, so in there you have 5-minute cadence, and 2.5-minute in the rush hour.

I live near both stations, S-Bahn is the quicker way to go to my office, but being the station in the outskirt, I need to know when trains are coming.

I could be mistaken, but what OP is mentioning might be closer to Renfe rodalies/cercanías than the metro (U-bahn). Maybe somebody from Munich can elaborate. Surely U-bahn is not running every 20 minutes in Munich, right?

you can do 2-3 minutes per train when every line has their dedicated tracks. with 7 lines sharing one track in the city center each train can't do better than 7*2 to 7*3, that is 14 to 21 minutes per train.