St. John's has not "officially" retired any men's basketball numbers. Several in the "Legends Club" are hanging from the rafters, but they are not retired. I am aware of three numbers that St. John's has retired in sports. Jack Kaiser's for baseball, and two players who were killed in seperate car accidents (incidentally within a year of each other I believe), one in Woman's basketball and one in Hockey.
Someone mentioned No. 13 being retired and I assume for Mark Jackson. I don't recall anyone wearing 21 after Malik Sealy and no one has won 20 since Chris Mullin, 13 was worn after Mark left by Barry Milhaven.
UNC as pointed out has retired numbers. According to Wikipedia a total of 43 jerseys have been honored and hang from the rafters, 8 of which have been retired.