At our gym the younger ones (up to age 11ish) usually have a 6:1 ratio (which is what I think works best) - beyond that (doesn't really matter what level) it is usually 8:1 because about 90% of the time not all attend training (we have day training, so school issues, commitment level, etc.). There are 8 officially in my dd's group, however one trains only 1 1/2 hours a day (mostly conditioning due to growth related issues), two others can't work their school schedules to daytrain, and on it goes - usually there are only 5 of them (on average) training at any one time. Works for my dd who is 110% committed and lives for the sport!
From conversations I have had with various coaches, they seem to prefer the 6:1 ratio.