Each team has 11 players on the field in an NFL game.
That means 22 players can be on the field during a normal live play: 11 on offense and 11 on defense. The offensive team has possession and tries to advance the ball, while the defense attempts to stop the drive, force a turnover, or prevent a score.
An NFL team uses far more than 11 players over the course of a game. Modern teams divide their personnel into specialized offensive, defensive, and special-teams units. Substitutions are allowed between plays, so a quarterback, linebacker, kicker, and wide receiver may all contribute without being on the field simultaneously.
The 11-player standard developed during the formative years of American football. In 1880, rule changes associated with Yale player Walter Camp reduced teams from 15 players to 11 and introduced the snap. A team that puts more than 11 players on the field risks an illegal-substitution or too-many-men penalty, making the number both a tactical standard and a formal rule.