When a penalty kick scores, it counts as a goal (just as any other goal). Penalty kicks that do not score (similar to regular goals) count for nothing.
The exception are kicks from the penalty mark (the "Penalty Shootout": the tie-breaker sometimes used when a game must have a winner, such as in most tournaments). When these kicks are successful, they are not counted as goals, but tracked separately to determine the winner.
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According to FIFA, at the end of both overtime periods, the players on the field are the only ones eligible to participate in kicks from the mark.
If one team has more players on the field, due to injuries or send offs, they must choose enough players to leave the field to even their numbers with the other team. This is often called "reduce to equate" and is only done this once; further injuries or send offs will not reduce further.
The remaining players are the only ones who may participate.
No substitutions are allowed, with one exception. If a goal keeper is injured during the taking of the kicks, and if the goal keeper's team has any substitutions remaining, then the goal keeper may be substituted for by a named substitute.
So, technically, anywhere from 2 to 22 players may participate in kicks from the mark.