To lose belly fat, all you have to do is crunches, is it right or not?

No,  you need to create a workout with cardio and strength training elements to decrease your overall body fat.

Crunches actually burn very little calories, it gives shape, harden the muscle and make them strong. Fat is normally stored on top of muscle, so you can have strong abs, but no one will see them unless you lose the fat on top. To do this, concentrate on a total body workout to decrease your overall fat content.