Official player limit
Any group size from two through twelve is supported. The developers do not name one ideal group size: fewer players make a more focused co-op experience, while larger groups can be more chaotic.
The three world choices
When starting a session, the host chooses one of three versions:
2-player worldFor a two-person minimum
3-player worldFor a three-person minimum
4+ player worldFor four or more
Some things in the world adapt to the selected version. Exact changes are not documented here.
The setting is not a cap
The world choice configures challenges. It does not limit how many players may join. If part of your group will miss some sessions, the official FAQ suggests choosing a lower setting.