Use a RMR (resting metabolic rate) calculator to estimate the amount of calories you burn at rest.
I don’t recommend you consume much less than your RMR. For an aggressive short term diet you can go about 10% under your RMR. However, never exceed 20%.
Your calorie deficit comes from exercise and non-exercise activity.
Note: BMR and RMR are slightly different. RMR measurements are typically taken under less restricted conditions. But close enough for us to use them interchangeably.