Why did David take so many stones?

Why did David take so many stones?

One tradition is that Goliath had brothers which David was supposed to take out at the same time he took out Goliath (1 Sam. 17:23-54).

It should also be noted that although he was confident that God would give him the victory over Goliath, he had probably fought off many animals as a shepherd for his father and was used to having extra ammunition at hand in case there was some surprise attack by other Philistines or in case he missed with his first shot.