How many calories do you burn in a 20 minute aerobic class?

It depends on the intensity and your body weight. Age matters too as the metabolism of youth ranging from 15 to 25 are significantly higher compared to the people age 30 and above.

If you are performing some sort of dancing and weigh around 80kg you will lose around 150 to 200 calories depending on your intensity.

Generally for every 3000 calories you lose, you will lose 1 pound of your body weight.

Hope this helps.