How many players does each team have on the field in an American football game?
Starting lineup
In American football since the 1950s, most upper level teams use a three-platoon system, each with its own starting lineup. The starting lineups for offense and defense, each with eleven players, typically get the most attention. The starting lineups are defined as the eleven players who take the first offensive or defensive play from scrimmage of a given game. (An offensive player does not have to be on the team that gets possession first to be considered a starter, nor does a defensive player have to be on the team that does not; the first play from scrimmage after the first change of possession counts as well.) The third platoon, special teams, in modern times is composed mostly of backup and reserve players from the offensive or defensive platoons, with the exception of the placekicker or punter; the players who, for instance, take part in the opening kickoff are typically not considered starters.
List of gridiron football rules
Teams get 3 timeouts per half, which can't carry over to next half or overtime, plus one per overtime period (same as in the NFL, but allows two per overtime period).
Although rules for the high school, college, and NFL games are generally consistent, there are several minor differences. The NCAA Football Rules Committee determines the playing rules for Division I (both Bowl and Championship Subdivisions), II, and III games (the National Association of Intercollegiate Athletics (NAIA) is a separate organization, but uses the NCAA rules).The National Federation of State High School Associations (NFHS) establishes the rules of High School Football. Two states, Texas and Massachusetts, use NCAA playing rules.