A match is played by two teams, with each allowed no more than 11 players on the field at any one time, one of whom is a goalkeeper. A match may not start if either team has fewer than seven players.
Official Competitions:A maximum of three substitutes can be used in any official FIFA match. The rules of the competition must state how many substitutes are nominated, from three up to a maximum of seven.
Other MatchesIn national ‘A’ matches, a coach may use a maximum of six substitutes.
In other matches, such as friendlies, more than six substitutes may be used as long as the competing teams reach an agreement on a maximum number and the referee is informed. If these criteria are not met, no more than six are allowed. The names of the substitutes must be given to the referee before a match, otherwise they are unable to take part.
When a team wants to make a substitution, they must inform the referee. The substitute must only enter the field of play once the player he is replacing has left and after a signal from the referee.
The substitute can only enter from the halfway line and during a stoppage in play. The player who goes off can take no further part in the match. If a substitute or substituted player enters the field of play without permission, he shall be cautioned for unsporting behavior.
Any of the players in a matchday squad may replace the goalkeeper so long as the referee is informed and the substitution is made during a stoppage.