There is legally no limit to how many you can have in a class or session. I would suggest that you just use common sense. How many people do you feel that you can safely instruct in a group class? I think if you have been teaching group classes for a while, then you can give verbal cues and focus on those needing extra help. It is completely different from a personal training client on how you would instruct them. I have also been to a class where it was clear that the instructor was trying to teach to many. I have also been in classes where I was surprised how well the instructor was able to keep everyone in order and good form. Also, don’t forget to take space into consideration.