How many calories are you supposed to eat while exercising to lose weight?

It should be 1200 as a total after exercise (so you should consume more). This is the very general figure that is recommended not to go under as the human body stops functioning properly. It is important to remember that everyone is different; the best general rule I have found is that consuming a total of 500 less calories than is required to maintain your body weight will cause you to lose weight at a constant and healthy pace. However, it can take some time to realise how many calories you should be eating to maintain your weight.

if you want extreme weight lose eat around 800 calories

This calculator will help you factor your numbers for a healthy, lasting outcome: http://www.cooks.com/rec/diet/