I burn somewhere between 500 and 800 calories in every BodyCombat class. Of course, it will depend on how hard you're working. If you are really interested in tracking calories burned, go ahead and purchase a heart rate monitor. It has been a great tool for me. I've lost 60 lbs in the last year, and BodyCombat has beeen a HUGE part of that. In fact, I love it so much that I recently became an instructor :)
As to how often you should do the class-- I'd recommend keeping it to 2 or 3 times a week. When you're trying to lose weight, cardio is important. But you need to be doing several types of cardio for optimal results and to protect yourself from injury!
Best of luck to you!