How many calories is to little?

I think I may have asked this question before about burned verse eaten.  Is it you do not want to have more then 1000 calorie difference or 500-750?  I just wanted to check.

You want to always consume at least 1200 calories as a female. Your calorie deficit (Difference between calorie expenditure and calorie intake) should never be more than 1000 calories a day and the optimal range is between 500 and 700 daily.

Thank You!!