Adam Clarke on Joshua 8:10

Numbered the people - ויפקד את העם vaiyiphkod eth haam, he visited the people - inspected their ranks to see whether every thing was in perfect readiness, that in case they should be needed they might be led on to the attack. There is no doubt that Joshua had left the rest of the army so disposed and ready, part of it having probably advanced towards Ai, that he might easily receive reinforcements in case of any disaster to the thirty thousand which had advanced against the city; and this consideration will serve to remove a part of the difficulty which arises from Jos 8:1, Jos 8:3, Jos 8:10, collated with other parts of this chapter. Had he brought all his troops in sight, the people of Ai would not have attempted to risk a battle, and would consequently have kept within their walls, from which it was the object of Joshua to decoy them. See the preceding observations, particularly Jos 8:10-12 (note).