Minimum Daily Calorie intake

It is difficult to set absolute bottom calorie levels, because everyone has different body composition and activity levels. Health authorities do set some baselines - these are 1200 calories per day for women, and 1800 calories per day for men. This doesn't really make too much sense - are you are sedentary person with little muscle mass? Or someone who is tall, muscular, and exercises a lot? Absolute levels don't work - but do give us a starting point.
When reducing calories:

Try not to lower your calorie intake by more than 1000 calories below maintenance. Doing so may invoke the bodies starvation response, which can lead to the Yo-yo dieting effect.

Try to gradually lower calories. A sudden drop (such as 500 calories or more) can cause your metabolism to slow.

What happens when calories are too low?
1)Muscle mass is broken down for energy (catabolism).
2)Metabolic rate will begin to drop (typically) after 3 days of very low calories - this is related to, and compounded by the loss of muscle mass.
3) With very low calories you risk sluggishness, nutritional deficiencies, fatigue, and often irritability. You are completely set-up for a regain in fat if you suddenly return to your previous eating patterns.