The answer to your question is to calculate your BMR, or basal metabolic rate. It’s an approximate calculation based on your sex, age, height, and weight. Your BMR is the amount of calories you need to maintain life when completely sedentary. If you only eat that number of calories, and do ANY physical activity, you will begin to lose weight as your body taps into your reserves for energy.

There is more nuance, but the simple math is calories in vs calories out. If you have a deficit, your weight will decrease. People counting calories to lose weight generally target their BMR minus 200 to 500 calories daily, though 500 is a bit much in my experience.