What did God command Moses to take of Israel's men in Numbers 1?

Answer: A census

God told Moses and Aaron to number every man twenty years or older who could serve in Israel's army.

““Take a census of the whole congregation of Israel by their clans and families, listing every man by name, one by one. You and Aaron are to number those who are twenty years of age or older by their divisions—everyone who can serve in Israel’s army.”— Numbers 1:2-3 (BSB)