You must have at least three letters of recommendation submitted by professors or others who can assess the quality of your academic performance and scholastic potential. Professors who have taught your advanced technical courses might speak effectively about your scholastic potential. A professor who supervised you on an undergraduate project might speak effectively about your creativity or initiative. If you are currently in the work force, a current or former supervisor might speak effectively about your initiative and ability to do complicated tasks. A recommender can also give insights into any difficulty or misstep you might have overcome. Choose your recommenders so as to provide as complete a picture as possible of you. Too many letters will not help your case, especially if they make similar points. It is an extremely rare case that needs more than five letters.