Most organizations recommend women eat 2000 calories a day, and men 2500. But of course, this is just an average. How many calories you need depends on your body weight and activity level.
Use this simple tool to find out how many calories you need to stay at your current weight. If you want to lose weight: eat less. If you want to gain weight: try to eat more.
There is one caveat: this tool does not give accurate results for very muscular people (it will underestimate your needs) or obese people (it will overestimate your needs).