Because we have many more kids in compulsories than optionals, and because it's a little easier to run a compulsory practice than an optional one because everyone is doing the same things, we have much larger compulsory than optional groups. The ratio when I was a level 4/5 was about 12:1 or 14:1. It got a little dicey on beam, because you had to wait for your turn for a while, but the coaches found side stations for us to work on and we always stayed busy.
In optionals, we have a ratio of usually 8:1. Sometimes we combine our two groups and it's 15:2 or 16:2, which works fine in terms of coaching but creates a problem with the equipment. In that situation we have 4 different ways the vault needs to be set or more, 4 girls for each high beam, and issues with the bars because some girls need them in close and some need them out far. Tumbling's a mess, obviously. Luckily we don't have to do that often.