Unless a current player already wearing the number; if a number is retired by the franchise, no player in that franchise can wear that jersey number and if the jersey number is retired by the league no one can wear that number again in the league. for example, no one in MLB can wear 42 because of Jackie Robinson except Mariano Rivera because he already wore the number before it was retired.
The first number in professional sports to be retired was the number 1. It was worn by Ray Flaherty of the New York Giants football team, and was retired in 1935.
1
Kavon Frazier
Joe Montana
MARTIN BRODEUR
Reggie Jackson
football
Allen Iverson ans Dale Ellis.
There is no exact number as anyone with a sporting ability is classified as a sports person.
Drew Brees, Matt Hassleback,and many more.
By saving the players jersey and allowing no one to have the number
Nemanja Vidic and Peter Crouch (when he was on Liverpool)