It’s highly variable and you should probably just eat when you’re hungry unless you gain excessive amounts of weight following that strategy. A highly active male of 15 will typically burn 2700+, while a highly active female of 15 will typically burn 2300+, so you probably need MORE food, not less. If your BMI is less than 22 or 23, you REALLY shouldn’t worry about it that much, and there’s an even wider margin if you do muscle-building exercise too. Basically, it sounds like you’re just being self-conscious. I would suggest eating at comfortable levels unless you actually weigh yourself and find you put on more than a few pounds in a month. At your age, you’re still growing and weight gain is inevitable.