Trumpeters can be one or two players as well as in larger groups. All other groups (renaissance band, patriotic band, etc) can be as few as three. Four is better. And of course more is still better.
The Renaissance band does very well at 5 players.
The Dixieland band does very well at 6.
The Patriotic Band does very well at 12.
The Trombone ensemble is good at 4.