For example, a local football team wants to enter a tournament. However, they need 11 players in order to enter, but they only have 9. Those 9 players would probably be asked to talk to all their friends and see if any of them would be prepared to play. They need 2 more people in order to have a team of 11, so the friends would be asked if they could help to "make up the numbers" (to help the team achieve the number of players required).
In the example you gave above, they're warning you that instead of simply watching the wrestling competition, you might actually be asked to join in if there aren't enough locals (or of course they might simply want the amusement of watching tourists trying to wrestle!)
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