Jim, serious question for you:
What ratios would you think appropriate? Not just for dry suit, but open water or courses that are no more challenging than open water for a student new to the skill.
In situations where I've had more than one buddy team and a buddy lost buoyancy control (upward), I've done a very rapid "stop, think, act" and assessed whether I was confident the diver could correct the situation on their own. If not, step 2 was "is my DM intervening to correct the situation?" Some DMs I work with a lot I can pretty much count on. Others I know before I look I probably can't. If none of the above, I intervene and give the rest of the group the thumbs up.
Not saying this is a good thing: My preference is to teach groups of 2-3 at a time. When the shop gives me a class of 8, I'll usually break them up into two groups of 4 for the dives.
For OW, 2 to 1 if the instructor has confidence that the students are good at that. I never took more than 2 students at a time on the 1st two OW dives. I had a family of 4 and the 1st day I did 4 dives to their 2. One pair at a time. Satisfied that they were good to go, the next day I switched the buddy pairing and took each team one at a time and on the 4th dive the all went. That was the last time I did an OW class that big. It went fine but was a lot of work. After that my OW classes were either private or 2 to 1.
For AOW 2 to 1 or 1 to 1.
UW Nav, I did a class of three but one of the students was also a certified instructor who was getting some additional con-ed for her college classes.
Tech I have had teams of three and one to one for AN/DP
Drysuit, 2 to one max. Usually private though because I had unlimited pool access.
I did take three divers one time for a drysuit OW portion after having had all of them in three pool sessions and was confident in their ability. I called the dive immediately upon descent. Vis was near zero and it wasn't safe. Finished them up individually after that due to scheduling.
The only class I had more than 2 after that were the rescue classes I ran. But those were always tightly controlled and I had assistants and surface support. And most of the stuff is surface work. The underwater stuff was always with a two to one ratio.
I retired last year and I can't imagine any class now with more than a 2 to 1 ratio except for some portions of rescue. Or with a good solid team of 3 for tech.
It really does come down to how many hands do I have. Sometimes 2 is enough for one student.
I'm fortunate that I was selective with who I took on. My own OW instructor had one student that two hands were barely enough to control them.
