How many players can be on the field at once?
3-7-1: Between downs any number of eligible substitutes may replace players. Replaced players shall leave the field within three seconds
3-7-4: During a down a replaced player or substitute who attempts unsuccessfully to leave the field and who does not participate in or affect the play, constitutes an illegal substitution.
NOTE: Participation by a replaced player or substitute is illegal participation as in 9-6.
9-6: ART. 1 . . . Prior to a change of possession, or when there is no change of possession, no player of A or K shall go out of bounds and return to the field during the down unless blocked out of bounds by an opponent. If a player is blocked out of bounds by an opponent and returns to the field during the down, he shall return at the first opportunity.
ART. 2 . . . During the down, no player shall intentionally go out of bounds and return.
ART. 3 . . . No replaced player, substitute, coach, athletic trainer or other attendant shall hinder an opponent, touch the ball, influence the play or otherwise participate.
ART. 4 . . . It is illegal participation:
a. When any player, replaced player, substitute, coach, athletic trainer or other attendant enters and participates during a down.
b. If an injured player is not replaced for at least one down; unless the half time or overtime intermission occurs.
c. To have 12 or more players participating at the snap or free kick.
d. To use a player, replaced player, substitute, coach, athletic trainer or other attendant in a substitution or pretended substitution to deceive opponents at or immediately before the snap or free kick.
e. For a player to be lying on the ground to deceive opponents at or immediately before the snap or free kick.
f. For a disqualified player to re-enter the game.
You can have 22 in the huddle, as long as the 11 substituted for leave within 3 seconds.