How many calories do I need a day to keep my metabolic rate from dropping?

It can't be less than 1500 calories that you normally metabolize, otherwise your body won't work enough, and how can there be extra consumption? If you don't consume, you can only hoard! As for your daily calorie intake between 600-800 calories, or even 400-500 calories, it is absolutely unacceptable. Be healthy. Don't lose your health just to lose weight quickly, it is not worth the candle!