Can you tell me how much weight will I lose eating 600-800 calories a day?

Restricting calories. Below base metabolic rate (wt in pounds x 10) triggers starvation mode for the body causing you to overeat when food is available. You may lose a little weight at first but then the body will try to store fat to fight starvation. You'll gain it back and it will be harder to lose. Best to work with a nutritionist/health coach on a life plan for steady healthy sustained weight management.