My max for several years was 15/wk. Much depends on the type of classes and the instructor’s physical involvement and age (lol). For some of us who have been teaching for decades, it is much more feasible to teach aqua, mind body, cycling and low impact varieties at higher frequencies than it would be to pack the schedule with high-impact options like we did in our twenties. I’ve seen peers of all ages who have suffered overuse injuries or professional burn out under the weight of an over-committed schedule. So, my philosophy is do what you love, give it your best AND do what it takes to take care of yourself for the long haul!