No one can wear 21 because it was retired for Michael Briere
Roberto Clemente wore uniform number 13, and 21 during his first year in 1955 with the Pittsburgh Pirates, and wore uniform number 21 the rest of his career from 1956 -1972.
Steelers starting quarterback Ben Roethlisberger wears #7.
He wore #21.
Robert Golden is number 21 on the Pittsburgh Steelers.
Rafael wore jersey number 21 for mancunian republic in 2010-2011.
He wore #21.
Borje Salming wore number 21 for the Toronto Mapleleafs. However, he left the team in 1989 and his number 21 was taken by a future NHL star named Sean McKenna.
According to Baseball Almanac, no one on the 1995 White Sox roster wore #21.
No Steelers player wore #6 in Super Bowl XIII, which was played on January 21, 1979. The #6 was worn by Jim Elliott in 1967 and then again by Bubby Brister from 1986-1992.
In 1980, running back Clark Gaines wore #21 for the New York Jets.
From what I've researched, no one wore number 21 for the San Diego Chargers in 1998.
Pac Man Jones wore # 21 in 2008.