How many smooth stones did David choose from the brook before facing Goliath?

Answer: Five

David took his staff, put five smooth stones in his shepherd's bag, and with his sling in hand approached the Philistine.

“And David took his staff in his hand, selected five smooth stones from the brook, and put them in the pouch of his shepherd’s bag. And with his sling in hand, he approached the Philistine.”— 1 Samuel 17:40 (BSB)