Joshua 8:3

So Joshua arose, and all the people of war, to go up against Ai: and Joshua chose out thirty thousand mighty men of valour, and sent them away by night.

So Joshua ... chose out thirty thousand men of valour - Joshua despatched 30,000 men, under covert of night, to station themselves at the place appointed for the ambuscade. Out of this number a detachment of 5,000 were sent forward to conceal themselves in the immediate precincts of the town, in order to seize the first opportunity of throwing themselves, into it.