American Football NCAA Rule 7-1-4-a

a. Formation.
At the snap Team A must be in a formation that meets these requirements:
1. All players must be inbounds.
2. All players must be either linemen or backs (Rule 2-27-4, A.R. 7-1- 4-VIII).
3. At least five linemen must wear jerseys numbered 50 through 79 (Exception: When the snap is from a scrimmage kick formation, par. 5 below.)
4. No more than four players may be backs.
5. In a scrimmage kick formation at the snap (Rule 2-16-10) Team A may have fewer than five linemen numbered 50-79, subject to the following conditions:
(a) Any and all linemen not numbered 50-79 who are ineligible receiver(s) by position become exceptions to the numbering rule when the snapper is established.
(b) Any and all such numbering-exception players must be on the line and may not be on the end of the line. Otherwise, Team A commits a foul for an illegal formation.
(c) Any and all such players are exceptions to the numbering rule throughout the down and remain ineligible receivers unless they become eligible under Rule 7-3-5 (forward pass touched by an official or a Team B player). The conditions in 5(a)–5(c) are no longer in effect if prior to the snap a period ends or there is a timeout charged to the referee or one of the teams.