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; and make not all the people to labour thither; for they are but few.

Let not all the people go up ... for they are but few. Since the population of Ai amounted to 1,200 (Joshua 8:25), it was a considerable town; though in the hasty and distant reconnoitre made by the spies, it probably appeared small in comparison of Jericho; and this may have been the reason of their proposing so small a detachment to capture it.