Oh! I wonder if the US census data could help you with this. Doesn’t that have questions regarding the occupations of the people surveyed?
That’s going to be a bit under-reported, in my opinion, because people would be more likely to list their full time jobs than their part time jobs and a lot of fitness instructors are very part time.
Interesting question, though. Some companies (those that are aggressively trying to show growth) will happily give you the statistics for how many people are licensed or certified with them. For other companies, this is proprietary information and although they keep great details, they won’t share it.