Uniform number in Major League Baseball (Major League Baseball) In baseball, the uniform number is the number that each player and coach wears on their uniform. Because no two persons from the same team may wear the same number, numbers are used to readily identify each person on the field. Although intended solely for identification,... There are no more fresh baseball jersey numbers in the world—the final unused big league uniform number has already been claimed. On August 20, the Yankees called up Miguel Yajure and assigned him No. 89, the only number from 0 to 99 that has never been worn in a Major League Baseball regular-season game. The Philadelphia Phillies are the only major league club that wears a number on their sleeve. The White Sox accomplished it from 1971 through 1975, and the Cardinals did it in 1979 and 1980. Baseball's official regulations say that all team players' uniforms must be similar.