How many crunches do you need to do to lose 8 ounces of belly fat?

There is no such thing as spot reduction. You cannot reduce fat only in your abdomen by doing crunches. Unfortunately, that is not how it works. You must lose total body fat to have less fat in your stomach, so burn more calories than you're taking in. Crunches help burn some calories, and help define your stomach muscles, but will not burn fat just in that area.