It all depends on whether you are under, over or normal weight now. This is determined by your body mass index (BMI), which your doctor can easily calculate on a chart by using your height and weight.
If you are average weight you should gain 25-35 pounds (I tell my patients to stay more toward the 25 rather than the 35 pound range so that there is less to lose after they have the baby).
If you are underweight you should gain 28-40 pounds, and if you are overweight you should gain 15-25 pounds.
Severe calorie restriction and fad diets should not be used in pregnancy, and if you have any questions you should speak to your doctor, who may also have you see a nutritionist.