[Will]

At Apnea Academy we have had courses with up to 30 students - the Italian Instructors course I was on had 64! Most are a lot smaller though.
HOWEVER. The difference is there is a maximum ratio of 4 students per instructor, so even if it is a big course there are smaller sub-groups, and the best thing about it is that the instructors rotate, so that you benefit from the teaching of several experts.
For example in the AA instructors course during the course of the week you will be taught by Umberto Pelizzari and 8 other expert AA instructors on rotation. Not only this but also experts in medicine, biomechanics, yoga, communication, psychology, diet and OTR all give lessons. The different teachers compliment each other and give you a holistic training.
As I understand, in all other freediving teaching agencies an instructors course is run by one or maybe two instructors. Although I don't doubt their expertise I think that any single freediver will have holes in their learning or methodology, which could ultimately limit their students in one or several aspects. For beginners this won't be so evident, but I think in an instructors course it can sometimes become an issue.

Anyway, I'm an AA instructor, so obviously biased as well!