rules

Question
how many players do you need to have to start a game?

Answer
Richard,
Thank you for your question.  Rule 1 addresses the number of players permitted to play.  The game shall be played between 2 teams of not more than 11 players each. A team legally may play with fewer than 11 players, but a foul for an illegal formation occurs if the following requirements are not met:
1. When the ball is free-kicked (Kick Off) there must be at least four (4) Team A players are on each side of the kicker when the ball is kicked (Rule 6-1-2-c).
2. On scrimmage plays, at the snap, there must be at least five (5) players wearing jerseys numbered 50 through 79 on the offensive line of scrimmage and no more than four (4) players are in the backfield (the exception is to the number requirement 50-79 is on scrimmage kick formations).

So to answer your question, technically a team starting the game by kicking off can have no fewer than 9 players.  From an officiating standpoint (a principle, not rule) a game should never start unless there are the 11 players on the field.

I hope this answers your question.
Victor Winnek