I have never seen a standard, and I think it would be impossible to control because instructors can teach at multiple locations plus could even teach on their own in small groups outside.
I assume that this instructor is teaching all those classes at your facility and I am wondering why – from a business perspective – you would let this happen. Even if this instructor were to be the most popular one on the planet, you are exposing your business very much if this person all of a sudden could not or would no longer wish to teach for whatever reason.
Another thought: some instructors have a propensity to over-training bordering on a compulsion to exercising. If this is an employee, you may want to find a way to address it.