in my personal experience the minimum is 2 but 3 is better... then with 4 you have a good fat group of players, and 5 is the functional maximum and might already be too much depending on STUFF*.
Anything more makes the game too slow, fragmented and chaotic, although I've heard of people claiming to be able to run 6+ groups "with satisfaction" (your mileage may vary :P )
*STUFF being the personal ability & expertise with AW of the MC, the general behaviour and attitude of ALL the players, the relative calm of the location where you play, etc.
The more people you have at the table, the more time each will have to WAIT to play, while their PC is not in the current scene.
Also, the more people you have at the table, the more DISCIPLINE they need to show in order to not have the session degenerate into an unpleasant chaos.
Remember that AW is supposed to be a highly immersive and emotional game, punching you in the gut with harsh decisions.
The more people at the table, the harder it is to have everything run smooth, feel personal, and to keep the general mood and immersion at the right tune.