How many calories should be eaten of a person weighing 50 kilograms?

That depends: If the person is physically active and trying to gain muscle mass, plenty of calories and plenty of lean protein. A small-bodied triathlete may weight 50 kg and require 5000 calories per day. The daily caloric need for a sedentary person is who only walks a bit 30 x weight in kg, or 1500 kg. Please read a good guide to energy expenditure and its effects on caloric needs.