I've run classes up to about 25 at a time. That's usually groups of 2-3 (art is primarily grappling), and groups are given assignments to work on (not usually the same for the whole class, because of range of skill levels). They don't need a lot of attention, so a single instructor can handle it. But 10 groups was about the upper limit of what we could fit in the space at the place I started teaching. We could get a few off-mat for striking drills, but there wasn't much space off-mat. I think more than 10 groups (including the folks off-mat) needs another instructor, both because of the time requirement, and because it's difficult to stay positioned to see all of those different groups easily for safety and coaching purposes.I agree with there being a lower limit. I personally would not want more than 10 students unless I have an assistant instructor.
I've been in larger classes (when visiting other dojos), and yeah, they always had help. I suppose more could be done with line drills (kata, traditional striking drills, etc.), if everyone is on the same drill.