It used to be the substitute number when teams were only allowed to use 1 sub. Outfield players wore numbers 1-11 and sub wore 12. When more than 1 substitute started being used in the game it became just another squad number.
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For International football there is no set numbers, you could be number 7 one game then the next game number 2. It is usually the right midfielder who gets the number 7 shirt for England.
Since transferring from Fulham in July 2010, Chris Smalling has been wearing the number 12 jersey at Manchester United.