Basically if he has number X on his cap, it means he is 
  the Xth player to play for his country in that form of 
  the game.   

Dianne van Dulken:
  [Steve Waugh]  had the brilliant idea a few years back
  of getting it on their caps, so they are in touch with 
  their place in history, etc, etc.

Mike Holmans:
  I notice that both India and England have followed suit.