You need at least three letters of recommendation. At times people submit four, but some chairmen will pick three at random and just reads those. One of the letters you submit must be a letter from the chairman of urology at your school. Without this letter, your application is severely weakened and every program will ask why you lack this letter. If you cannot obtain one from your home chairman, a chairman at a program where you did an away rotation will at least reduce some of the damage. Your other two recommendations need to be from people that know you well and would be willing to go to bat for you. If you have done any research, one of the letters could be from your research advisor. There is no doubt that it is of great help if the people writing your letters are well-known, but a lukewarm letter from a big-wig does not stand up to a very strong letter from a lesser known (or even unknown) clinician. Most people get all three letters from Urology faculty, while others get one or two letters from people in other specialties (surgical specialties are preferable). It probably matters little--just make sure you have the chairman letter and the other two are from people that know you well. If you do an away rotation early enough in the application process, it is advisable that you get one of your letters from this institution. A strong chairman letter from an away rotation can be a considerable addition to anyone's application.