In my personal training business, I only train one-on-one. I teach MELT courses at my studio for 2 or 3 people at a time. When I teach this course at an affiliated facility, I take up to 8 people.
I also teach MELT and water aerobics as a group exercise instructor, and there are always large groups (at least 10 and sometimes for water over 30).
Since my business is multi-faceted, I carry 6-10 personal training clients at a time. That leaves me time for my coaching groups, which number anywhere from 10-30 people at any given time, and for the productions I promote and direct, and time to run the business part of the business.