Answer:
I think you're asking why David got five smooth stones to fight Goliath and only used one.
The Bible does not state specifically, but he may have wanted more ammunition in case he missed or in case some of the Philistine soldiers did not honor the terms of the fight and came to join in.
Many people use II Samuel 21:15-22 to point out that Goliath had 4 brothers that were also giants, and the four extra stones may have been for them if they had shown up. The brothers were: Ishbibenob (killed by Abishai), Saph (killed by Sibbechai), an unnamed brother (killed by Elhanan), and another unnamed brother (killed by David's nephew Jonathan).
2 Samuel 21:15-22 Moreover the Philistines had yet war again with Israel; and David went down, and his servants with him, and fought against the Philistines: and David waxed faint. And Ishbibenob, which was of the sons of the giant, the weight of whose spear weighed three hundred shekels of brass in weight, he being girded with a new sword, thought to have slain David. But Abishai the son of Zeruiah succoured him, and smote the Philistine, and killed him. Then the men of David sware unto him, saying, Thou shalt go no more out with us to battle, that thou quench not the light of Israel. And it came to pass after this, that there was again a battle with the Philistines at Gob: then Sibbechai the Hushathite slew Saph, which was of the sons of the giant. And there was again a battle in Gob with the Philistines, where Elhanan the son of Jaareoregim, a Bethlehemite, slew the brother of Goliath the Gittite, the staff of whose spear was like a weaver's beam. And there was yet a battle in Gath, where was a man of great stature, that had on every hand six fingers, and on every foot six toes, four and twenty in number; and he also was born to the giant. And when he defied Israel, Jonathan the son of Shimea the brother of David slew him. These four were born to the giant in Gath, and fell by the hand of David, and by the hand of his servants.