Q: how many calories does one sit up burn?

A: Honestly I don't know, but very very few. Maybe a quarter of a calorie? But no matter what the answer is, trying to sit-up your way to losing fat is far far from the best way. The big advantage of weight training is that if done properly it will burn calories around the clock as your muscles recover. By limiting yourself to just the sit-ups, you ignore 90% of your muscle mass, thus ignoring 90% of potential fat loss. Please read this, read the other articles it references, and reconsider your training and diet. Good luck!