I have had varying degrees of experience in terms of the amount of people in my cake classes; once it got to the point where we had about 20+! And there was one class where there was only one other person, another where there were two other persons, etc.
The one with twenty, the instructor rarely looked at individual progress, or lack thereof.
How many is too many?
It depends on the subject and the instructor. I took a class with Nicholas Lodge and there were 30 in the class. It was long enough, and his instructions were clear enough that it was just fine.
The early Wilton classes I took were limited to 10, mostly because of space.
I don't think that there is a "magic number". I think, though, that as long as you are learning something (preferably more than "one" something) in each class, then it doesn't matter how many/few people there are.
I tought a decorating class tonight with a group of eight. first time for me...after about 3 hours, i couldnt believe how little we got done. i mean, i know it takes awhile to make cakes - i usually spend AT LEAST ten hours on an average cake, but what we did tonight was make buttercream, level, torte and fill, then crumbcoat and very scantily decorate the tops. i felt like i hardly got to walk individuals through steps or to see their final results because there were so many of us. now, it may have just been ME...but maybe it was just too much for SERIOUS BEGINNERS (!!!) ...but i think that just a handful would be perfect - 3 to 5, maybe it gets easier for the instructor to teach more people as there is more experience. OR- it also may be different in like a wilton class because i think that they have instruction manuals....we did not, this was a fresh batch of i havent a clue of anything individuals so.....?........
I don't know why I'm asking I couldn't answer you question anyway.
i know, its late and i am just on here, no real reason, just becaus- there's not much going on tonight with the galleries down and i should just go to sleep but instead maybe i'll stick around to poke fun at smartbutts like nick and smart alecs like janette. ( 
just havin' fun of course )
In any case, I guess. 
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