David girded his sword on his clothing, and he tried to go; for he had not proved it. David said to Saul, I can't go with these; for I have not proved them. David put them off him.
Some students of this passage have suggested that David chose five stones because Goliath had four brothers, and he wanted to be ready to attack them too. However there is no indication in the text that David had any concern for them or even that they were present at this battle. He probably chose five stones simply so he would have some in reserve if his first shot missed its mark.