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.
You might want to check some of the US government websites (.gov)for that. I’d start with the Dept. of Labor & Census Bureau. The term “fitness instructor” is pretty broad. To tell you the truth, I haven’t heard that term in a long time – I know that’s what we were called in the 80’s & early 90’s. Then the terms “personal trainer” & “fitness professional” came into vogue. I actually classify myself as a “fitness consultant” on my tax returns. Just curious – why do you want to know, is it for a project or paper?
There were about 251,000 fitness trainers employed in the US in 2010 via http://careerplanning.about.com/od/occupations/p/fitnesstrainer.htm
Hope this helps,