Some presume that David took five smooth stones instead of just one because he had some doubt. However, there is no indication in the story of David and Goliath that by picking up five stones instead of one that David was doubting God. Rather, David was simply being prepared. What if the Philistines attacked him after he killed Goliath? How would he have defended himself? David was simply being prepared when he took the four additional stones. Also, he couldn’t have known that one stone would be enough to kill the giant. God had not promised that David would kill Goliath with the first stone.
David had experience in defending the sheep he guarded with his sling and stones. It would seem that the animals David had faced were far braver than the men with Goliath, because they all turned and ran away (1 Samuel 17:51). David told Goliath that he (Goliath) came with spear and sword, but his weapon was God the Father (1 Samuel 17:37). He trusted God with all his heart, believing that God would tell him exactly what to do and how. And so He did.
Others speculate that David chose five stones because Goliath had four brothers, but this is without biblical basis. There is no reason to believe that David thought if he killed Goliath that he would have to fight the giant’s brothers. Further, the Bible does not specifically say that Goliath had four brothers, although he had at least one (2 Samuel 21:19). David’s faith was in the Lord, and he knew from experience God’s faithfulness. David’s faith was born out of his experience of God’s grace and mercy in his life up to that point. The Lord had delivered him out of dangerous situations in the past, proving His power and trustworthiness, and David relied on Him to deliver him from the Philistine. Whether it took one stone or five, David recognized that the power was not in his sling, but in the Lord of hosts. As David wrote later in Psalm 21:13, “Be exalted, O LORD, in your strength; we will sing and praise your might.”