For International tournaments (such as the World Cup), it is necessary to call up a certain number of players - usually 23 - to the national squad. In these cases, squads cannot contain more than, or less than, the stated number of players.
This is unrelated to the number of players you have picked in the team. There are 11 first team slots plus 9 substitute slots available to pick, so you will usually need to have 3 players called up who are not selected in the team.
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