Wondering,
Gemini helped us learn how to spacewalk, change orbits, rendez-vous and dock and spend weeks in space. There were 2 astronauts.

Apollo had a LM to land on the Moon while the CM stayed in orbit. The LM had 2 astronauts. There were 3 all told, but only because another vehicle had to be manned.

Titov and Manarov spent a whole year aboard Mir, performing spacewalks too, and the other things. They were only 2 cosmonauts (except for visiting crews).

Mir has many times only been manned by 2 cosmonauts.
ISS is currently manned by 2 cosmonauts and it doesn't seem to be an issue anymore.

The shuttle was manned 4 times by only 2 astronauts. Of course, it wasn't meant to be used that way, but it's not impossible either.

Makes me wonder: do we need more than 2 astronauts to go to Mars (assuming the entire crew lands on its surface)?
It's psychologically easier to deal with only one other person than 2 or more, in a spacecraft. There is far less room and oxygen and food and water needed than with 4 astronauts.

If there's a crew of 4 and one is incapacitated, it will as well affect the rest of the mission.

Two astronauts...the minimum possible for any envisageable flight.

Raoul