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.
If your current national squad contains the wrong number, you will be told the required number when attempting to confirm your squad in the News message. You will need to remove any excess players or call up additional players if you do not have the correct number in your squad.
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.