I'm a little confused - is it 14 max if they were all to show up (probably not in the summer), or is an average of 14 everyday accounting for those on vacation? I think they need a little better advance planning to account for the numbers week to week of how many kids to one coach there will be, but if there is a fairly lax time off policy in the summer then it probably could well be true that it will be an average of 10 girls per day, which I would say is about the high end of the acceptable ratio for that level/age. Smaller (maybe 1 to 8 or 9) would be ideal especially because it would give more ability to spot turns before getting tired, for example, but 1:10 isn't unheard of at that level.
One time I was in a group about that high of a ratio (higher, even), due to a coaching "emergency" of sorts where someone left before they could be replaced, and it took several months to find someone. There were some other coaches sort of in and out, but primary responsibility was falling on one person for all levels 8-10. It worked out fine because we had a very good coach, but it was very stressful for the coach. We all got our skills but it wasn't ideal. I wouldn't say at any point practice was unsafe, because the coach simply didn't allow that to happen, and it was very productive compared to lots of gyms I've seen. However I worked with that coach as a beginning optional in a MUCH smaller group and her productivity was through the roof then (seriously, it cannot even be described how bad on bars I was before and then how good on bars I was after). So I guess in that sense, it's more of ideal vs safe. And a lot depends on how good the coach is, as always, because a more lax coach who doesn't have enough drills and conditioning ideas could allow unsafe or unproductive progressions to happen.
Also level 7 to 9 is basically a huge difference skill wise (esp if we're talking about kids just moving up from level 6). When I was in a larger group it was minimum second year 8 with basic level 9 skills (if not routines) and then 9s and 10s. Beginning level 7s would not have fit at all. But I have worked in a program that had to do that - group 7 to 9 (except much smaller ratio so totally different).