Minimum of 3 players and 1 GM.
Maximum of 6 and one GM.
When I used to run the old STAR TREK role playing game (the FASA version) our rule of thumb was : Number of players should equal the number of characters that could be beamed up or down from a standard tRasport Room.
That worked out to 6 characters.
Also I tend to add 30 minutes of playing time required for any group larger than 3 or 4 players. That way each player gets enough time (potentially) in the spotlight.
- Ed C.