That is a good question, but it requires a little more information for someone to answer.
How much do you currently weigh?
How tall are you?
How much do you want to weigh?
Do you do cardio? If so, for how long each time you do it and how many times a week?
Do you lift weights? If so, how often do you lift?
Most CFers need 125-150% (someitmes more) of what the average person their age, weight and height needs (in calories). Protein is also a very important part of weight gain.
If you are asking this question specifically for yourself, the above questions could help me or someone else better answer your questions. If it is just a generalized question, go with the 125-150% more than the average person, especially if they are active and do some sort of weight training.
Julie (wife to Mark 24 w/CF)