Most of the department’s statistics come from a monthly survey of government and businesses. This is also how they get data on wages state by state.
Surveys are done by computer assisted phone.
I believe they categorize fitness trainers and aerobic instructors as one unit, and they are part of what is called recreational services. Moreover, they do not have a separate category for yoga instructors, so they are likely included.
You raise an interesting question about yoga teachers. If data is being pulled from a large gym I suspect they are likely to include yoga instructors among their fitness instructors. Many of the yoga people I know, particularly those who consider yoga as a healing art would not think of themself as a fitness instructor.
Natalie is correct that it would be difficult to get a really accurate count world wide. You could likely get a count of certified people by going through each certifying agency, but that would include people who hold a cert. but are not currently teaching, and it would not count people who have a degree but not a certification. You could probably get pretty close though.
And yes, I agree with you that that number is probably a little low.