I would think that a bare minimum of Three probably four is better with one of them being a surgeon who is multi talented perhaps in one of the other major areas like geology or piloting skills.

I have heard "somewhere" that missions planned on having the astronauts stay on Mars for a Year.

I don't think they would ever actually stay there that long but then again maybe it is practical because of the orbit takes you away from Earth.

So how would you like to be sitting there in orbit around Mars when it came time to make that major engine burn to break orbit and come back to Earth, and the engines malfunction?

Not much margin for error and it's a long way home!

Bet they have that one in the simulator.

OOps! I didn't get te engines shut down fast enough.

Let's see. Dump cargo? Sling shot around Mars?