5 -- Nomar Garciaparra
14 -- Jim Rice
21 -- Roger Clemens
42 and Mo Vaughn doesn't count because major league baseball had already retired it all across in honor of Jackie Robinson.
Now do you think that the organization will ever formally retire their numbers? In Nomar's case, he left on a rather sour note, so probably not. In Rice's case, he has to make the HOF, right? And as for Clemens, that depends on what hat he will wear when he is inducted into the hall-of-fame. It'll probably be with the Bombers, although Clemens has seemingly gotten less bitter about his "divorce" in '96 with us with every passing year.
I wonder if you HAVE to be a hall of famer in order to have your number retired. There are a lot of intelligent baseball posters here, so I'll leave that up to you.