You don’t have to approach every class you teach like it is your workout,. And really it shouldn’t be. You should focus more on the participants and not getting your own workout. I teach several different classes where I demo and cue, but I don’t get a workout compared to what I do when I am the participant in someone else’s class. It is your class, not just a group of people copying your workout.
This is where the ability to teach without participating can make all the difference in the world. When I put on my “educator” hat, I can instruct clearly and effectively without needing to get my own workout. It takes mental energy, but it doesn’t wear and tear my joints.
So, for me, my maximum in a day is 4 classes. 2 in the morning (one cardio that I’ll dance along with and one small-group that I’ll demonstrate but mostly instruct) and 2 in the evening (one small group that I demonstrate but mostly instruct, then a yoga class that is mostly restorative).
Normal class load for me = 14 classes a week, which gives me a good work/life balance.
I agree with all of the answers. The keys factors to consider in the answer to your question are to not use your class as your personal workout, take the time to give personal attention to your participants, and variety of class format to reduce over-stress to particular joints and soft tissues.
I can’t do more then one cardio class/day. But I can easily mix the day up with yoga, pilates or strength training type classes. These classes allow me to walk around and assist. Plus participants love that I’m helping them during class.