Since 1 June 2018 competitions have the option to permit the use of one additional substitute in extra time in all levels of football, whether or not the team has already used the full number of substitutes.
The number of substitutes, up to a maximum of five, which may be used in any match played in an official competition will be determined by FIFA, the confederation or the national football association except for men and women competitions involving the 1st teams of clubs in the top division or senior ‘A’ international teams, where the maximum is three substitutes.
NEW: Laws of the Game 2018/2019
Since 1 June 2018 competitions have the option to permit the use of one additional substitute in extra time in all levels of football, whether or not the team has already used the full number of substitutes.https://t.co/5Em9sHwceo pic.twitter.com/jcA5NGSOOU— FIFA.com (@FIFAcom) August 29, 2018
The competition rules must state how many substitutes may be named, from three to a maximum of twelve.
whether one additional substitute may be used when a match goes into extra time (whether or not the team has already used the full number of permitted substitutes)
Fourth substitutes were used in the 2018World Cup back in June .
OTHER MATCHES
In national “A” team matches, a maximum of twelve substitutes may be named of which a maximum of six may be used.
In all other matches, a greater number of substitutes may be used provided that:
the teams reach agreement on a maximum number
the referee is informed before the match
If the referee is not informed, or if no agreement is reached before the match, each team is allowed a maximum of six substitutes.
RETURN SUBSTITUTIONS
The use of return substitutions is only permitted in youth, veterans, disability and grassroots football, subject to the agreement of the national football association, confederation or FIFA.