How many defenders are there in a soccer match?

Any number of players may defend or attack at any time, except at penalty kicks (for defensive DFK fouls in the 18-yard box) and kicks from the penalty mark (tiebreaker used in tournaments requiring a clear winner). During those kicks, only the goalkeeper may defend, just as only one attacker may kick. Therefore, your answer is eleven during any time in the game other than PK or KftM.