This is MYTH.
500 crunches a day won’t give you the flat abs you crave…if you have an extra layer of fat on your belly! Crunches, which target your rectus abdominis muscle and obliques, will help build lean muscle mass and tone your abs, but they won’t reduce fatty tissue. Flat abs require a three-fold approach: eating a balanced healthy diet of fruits, vegetables, healthy fats and lean protein…increasing your aerobic activity, particularly with high-intensity exercise endeavors…and working your entire core (sure, crunches can be included) two to three times a week.
A Core Focus 
According to the American Council on Exercise, traditional crunches exert the muscles less than other types of abdominal exercises do. In 2008, researchers from the Biomechanics Lab at San Diego University found that the most effective ab exercises are those that involve body rotation and constant abdominal stabilization. For example, performing crunches on an exercise ball uses the rectus abdominis muscle 39% more than traditional crunches do, and target the obliques 37% more. Bicycle crunches activate the rectus abdominis muscle 148% more…and the obliques 190% more!
To build more lean muscle mass, which helps accelerate your metabolism and burn more calories, add exercises that involve your entire core, including your distal trunk (involving the shoulders and buttocks). Exercises such as plank and bridge will really round out your abdominal routine.
Burn the Fat
If you’re looking for flat abs, there’s just no getting around adding some cardiovascular conditioning into your fitness routine. Reducing belly fat isn’t just an aesthetic consideration. A layer of fat around the belly increases your risk for high blood pressure, heart disease, stroke, type 2 diabetes, sleep apnea, and certain types of cancer. High-intensity exercises, such as interval training are highly effective at burning abdominal fat.
According to the Centers for Disease Control and Prevention, 1 pound of fat is lost for every 3,500 calories burned. So, lowering your caloric intake by 500 calories per day can help you lose approximately 1 pound a week. Increase your aerobic activity and burn an additional 500 calories per week, and you can lose up to 2 pounds a week. However, the Mayo Clinic cautions against losing more than 2 pounds a week, as doing so can be unhealthy.
