Generally speaking at Cons, we have a minimum and maximum, but we break those all the time.

Missions is designed usually with 6 players in mind. However, since you never know what you're gonna get at a table ("Momma always told me Shadowrun Missions Teams are like a box of chock-o-lates!"), GMs are encouraged to adjust things on the fly to suit the team anyway.
At COns we've run with as few as 3 players, and as many as 10. Though this year we put in a "Soft" cap of 7 players, and a "Hard" cap of 8 (only if the GM wanted to run for that many, and only if all the pre-regged players agreed to that many).
Fewer tahn 3 does tend to be a little problematic, but is still doable with the right players. My original shadorun group where I played Bull the Ork Decker was primarily a two man team (with misc rotating players over the years), but our GM knew that and could plan for that. WHen there's fewer than 3 at cons, we tend to try and fold them into another game that still has room.
Bull