Each country is allowed a squad of 31 players. These squads are to be submitted to World Rugby by August 31 this year.
Once the squad has been submitted, a player can be replaced if injured, but would not be allowed to return to the squad.
There is also a stand-down period of 72 hours before a new player is allowed to take the field. Hence, a replacement player called into a squad on the eve of a game will not be permitted to play in that game.