In the NFC Divisional playoff game on 2018-01-14 between Minnesota and New Orleans, Minnesota's Stephon Diggs scored a touchdown as time expired in the fourth quarter to take the lead and thus win the game. Of course, the game does not end till the PAT is attempted. The players, media, coaches, staff, and security rushed the field. After about ten minutes, the officials were able to get everyone off the field and the Minnesota offense on the field. The Saints were not there, and it took another minute or so to get eight Saints onto the field and onsides before the Vikings could snap and kneel-down.
Now, why did they need eight Saints there? Why not seven? One? Zero? At what point does "eh, it's your fault you didn't have enough players on the field" become "this isn't football"? What rule specifies the minimum number of players a defense must have in order for the ball to be declared ready for play?
Note this is not really a question of having enough players on hand, lest they risk a forfeit; let's assume the players are somewhere nearby (sideline or locker room) but just are busy with something else.
The only relevant rule, 5-1-1, doesn't address it.
The game is played by two teams of 11 players each. ... Penalty: For more than 11 players on the field of play or the end zone while the ball is in play: Loss of five yards from the previous spot.
Note: It is not a foul if a team has fewer than 11 players on the field of play or the end zone when a snap, free kick, or fair-catch kick is made.