How many calories should active athletes eat in a day?

It Depends. It depends on your age, height and weight. It also depends on your goal. Are you trying to gain weight? Add muscle? Lose weight? Or tone?
Know your BMR. Learn your basal BMR at exrx.Net then you know what your calorie requirement is. Add according to your activity additional calorie so you will be able to build muscle and not lose lean muscle.