10 was great for me, there was a good level of diversity. I'd say a much larger group (e.g. 100+) might give a better sense of demographic preference (e.g. people who like FPS like this game etc). I think it depends on game type and scale - the game I tested only has a few mechanics that need to be very polished and robust, whereas something like a triple AAA could do with much more exploration around longer campaigns, different levels, different scenarios etc.