Jamieson, Fausset and Brown on Joshua 8:10

10. Joshua ... numbered the people--that is, the detachment of liers-in-wait; he did this, to be furnished with clear evidence afterwards, that the work had been done without any loss of men, whereby the people's confidence in God would be strengthened and encouragement given them to prosecute the war of invasion with vigor.

he and the elders of Israel--the chief magistrates and rulers, whose presence and official authority were necessary to ensure that the cattle and spoil of the city might be equally divided between the combatants and the rest of the people (Nu 31:27)--a military rule in Israel, that would have been very liable to be infringed, if an excited soldiery, eager for booty, had been left to their own will.