Albert Barnes Commentary Joshua 7:3

"And they returned to Joshua, and said unto him, Let not all the people go up; but let about two or three thousand men go up and smite Ai; make not all the people to toil thither; for they are but few." — Joshua 7:3 (ASV)

The total population of Ai was about twelve thousand (Joshua 8:25). It could therefore hardly muster three thousand warriors.