supply and demand
I am not aware of any specified minimum - and, in any case, it would be difficult to enforce. I delivered to an operator that only had 2 pilots, no cabin crew. Be hard to insist on a larger class size in this case. Don't forget, the requirement for CRMI accreditation is based on making the exercise valid in terms of letting the prospective (I) strut their stuff. 8-18 is generally held to be optimum ( and that is a fairly universal experience rooted in practice, not theory) but this is based on using the 'lesson' mode of delivery. I know of larger class sizes and was once asked to deliver to groups of 100. Didn't do that job but there is no reason why it shouldn't work. You just need to modify your approach. One of the most satisfying courses I ever ran was for 1 person. Crewing forgot to tell the rest.