Why did he take 5,000 men?
Rashi: This was an additional ambush, one closer to the city than the other.
Malbim: The first ambush was southwest of the city, far from the city (although he did tell them not to distance too much), and it was hidden from people of the city. This ambush stood by the camp north of the city, just it was drawn to the west. The next verse calls it the heel. It was to surround the enemy on two sides. It was not hidden at all; people of the city saw it, for it was by the camp near the city.