Thanks to YankeesNumbers.com, we know the answer. Charlie "King Kong" Keller wore number 99 in 1952 when he returned to the Yankees for 2 games after playing 2 seasons in Detroit.

Who's the only Yankee to have a jersey number in the 80s? This one is a bit more recent: Ryan Thompson wore 88 in 2000. No Yankee has ever worn a number in the 70s. Besides that, all numbers 1-69 have been worn by at least one player.

How many jersey numbers have only been worn by 1 Yankee? The answer is 4. The numbers are 4 (Lou Gehrig), 69 (Alan Mills and then Thompson and Keller.