How many calories are needed to build muscle will depend on whether you want abs from flabs, i.e. you want to convert fat into muscle or you are skinny and want to put on muscle mass. In the first case the calories required will be less than the second case. You have to eat twice the amount of calories if you are skinny to maintain your body weight and to build muscle. In the first case you have to eat lots of high water content food like fresh fruits and raw vegetables to provide necessary enzymes to the body which will help the body to burn fat and build muscle.
But whether you are fat or thin lots of calories are needed to build muscle, because you are adding something extra to your body. Otherwise if you keep exercising and eating less calories, you will face severe digestive and other problems like loss of energy, insomnia and stress. It will be difficult to face the day and its challenges.
But please do not fall into the trap of protein supplements to increase your calorie intake. They are for short term gains. Once you stop them the results are devastating. You may begin to lose the muscle you have gained and this might leave you feeling depressed.
So take the natural way. Eat lots of eggs, meat, chicken, legumes and dried fruits for strengthening your nervous system. Because you must be working out to build muscle. So your nerves should be strong or you may have a slipped disc or other nervous problems.
But here again I repeat the number of calories needed to buid muscle will be different according to the body weight. The calories needed should come from bean sprouts, skimmed milk, legumes if you are obese. Of course you have to take the animal protein if you are a non-vegetarian but in moderate amount. Like just one helping of chicken a day, one egg period. Also not more than 30 grams of nuts. But take lots of vegetable protein, oatmeal and other whole wheat products. Fibrous fruits and vegetables detoxify your system and make you burn fat so go heavy on these.
If you are skinny, the more the merrier. But the calories you need to build muscle should not come from junk food and colas but mainly from proteins and carbohydrates. Also take vitamins and minerals in the form of fresh fruits and vegetables.
Underweight people have to increase their extra calorie intake gradually. Otherwise the body will not accept a sudden spurt in calorie intake and they may feel sick and nauseating. So if you are taking an egg for breakfast take one and a half eggs for 3 days and then slowly increase to 2 eggs. Same with other foods.
Drink lots of water to flush your body and digest the extra calories you are taking. The calories needed to build muscle will have to be absorbed by your body and water is crucial for this absorption.
So eat more of the right kind of calories and you will be successful in building muscle.