I myself teach 15+ classes per week, with no issue. The trick is variety, and to remember that the instructor is there to guide and teach, not to get his or her personal workout. I became far more successful when I spent more time giving my clients personal attention during a group class, focussing on their needs and technique, as well as keeping them interested and entertained. Do this rather than doing every movement and rep with them at the front of the class. Walk around while you cue, make eye-contact, smile! A great instructor can do all of these things while they lead a fun, safe and effective class!