Who gave Moses advice about delegating leadership?

Answer: Jethro

Moses' father-in-law Jethro told him to appoint capable men over thousands, hundreds, fifties and tens to judge the people.

“Then Moses’ father-in-law Jethro brought a burnt offering and sacrifices to God, and Aaron came with all the elders of Israel to eat bread with Moses’ father-in-law in the presence of God.”— Exodus 18:12 (BSB)