Q: how many calories does doing crunches burn?

A: Please read http://www.mikesfitness.com/content/how-many-calories-doing-400-situps to understand why counting calories burned by crunches (or any other activity) is the wrong way to approach exercising and weight loss. You should do exercise for your whole body, not just your abdominals and also have a good diet to achieve good results. Doing hundreds of crunches will make no or little difference by itself.