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 smooth 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 smooth stones because Goliath had four brothers, and David was readying himself to dispatch all five giants. This theory is based on 2 Samuel 21:15–22. That passage lists four very large Philistines who were related to Goliath in some way: Ishbi-benob, Saph, Goliath, and an unnamed giant with six fingers on each hand and six toes on each foot. “These four were born to the giant in Gath, and they fell by the hand of David and by the hand of his servants” (2 Samuel 21:22). The account of 1 Samuel 17 does not suggest that David knew about Goliath’s brothers or that he believed he would have to fight the whole family. What we know for sure is that David and his men faced four other giants, somehow related to Goliath, in later battles.
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.”