Formations
The number of linebackers is dependent upon the formation called for in the play; formations can call for as few as none, or as many as seven. Most defensive schemes call for three or four, and they are named for the number of linemen, followed by the number of linebackers. For example, the 4-3 defense has four defensive linemen and three linebackers; conversely, the 3-4 defense has three linemen and four linebackers.
Read more about this topic: Linebacker