Understanding American Football Team Structure
Each football team has 11 players per side: 11 on offense and 11 on defense. Teams are allowed to play with fewer than 11 players (why would they want to do that?), but they're penalized for having more than 11 players on the field during play, which is also known as live action. In high school, three or more talented players may play both offense and defense. And a few rare athletes may play both offense and defense in major college football and the NFL. The nonstarting players (that is, those who aren't among the 22 or so players who are listed in the starting lineup) are considered reserves, and many of them are specialists.
For example, defenses may play multiple schemes employing a nickel back (a fifth defensive back) or two pass rushers (linebackers or defensive ends who are used strictly on passing downs to rush the quarterback). Also, an extra player is often used as the long snapper who snaps (hikes) the ball for punts, field goal attempts, and extra point attempts. Some of the reserves make the team because they're excellent special teams players who are great on punt and kick coverages because they're fearless tacklers in the open field.
The roster sizes in high school and college football tend to be unlimited, especially for home games. However, the NFL limits active, uniformed players to 45 per team. An additional player can be in uniform, but he must be a quarterback and enter the game only after the other quarterbacks have been removed from the game because of injury.
When this extra quarterback, or 46th player, enters the game, the other quarterbacks are deemed ineligible and can't return to that game even if they are healthy. A typical NFL game-day roster includes three quarterbacks, a punter, a placekicker, a kick return specialist, eight offensive linemen, four running backs, five receivers, two tight ends, seven defensive linemen, seven linebackers, and six defensive backs.