After defeating the Amalekites, what did David rule about the men who stayed with the supplies?

Answer: They would share the plunder equally

David said those who stayed with the supplies would share alike with those who went to battle, and it became a statute in Israel.

“When David came to the two hundred men who had been too exhausted to follow him and who were left behind at the Brook of Besor, they came out to meet him and the troops with him. As David approached the men, he greeted them,”— 1 Samuel 30:21 (BSB)