That’s a really broad question. I think it will depend on:
The size of the city
The number of members of that gym
The number of classes on the schedule
The time of day
How long that instructor has been teaching that class in that time slot.
I get 45-60 for one of my classes, which are great numbers for that club. But I used to teach the same format at a similar time at a club 4 miles away. That’s a large big-box gym with many more members, and 60 was a small class for me.