Good question, and for each person, there's a different answer.
For some people, they can do a 1200 - 1500 calorie diet, depending on height, weight, and activity level. For others, that kind of diet would be cutting far too many calories and could even be dangerous. And for others still, it may be too many calories.
Factors that contribute are height, age, activity level, and gender. Men can eat more than women. The taller you are, the more you need to eat.