How many calories should a 19 year old 85 pound girl eat a day?

To maintain weight: around 1500 (it depends on your height & activity level)

85 lbs for a 19 year old female is not healthy! A person @ this weight would have a very low BMI and be putting themselves at risk for many serious health conditions. Less importantly, 85 pounds for a woman is not attractive... studies show that a large majority of men are more attracted to an average or curvy figured woman than a super skinny woman.