Joshua 8:12

‘ And he took about five thousand men, and set them in ambush between Bethel and Ai, on the west side of the city.'

These were probably a further precaution rather than to increase the previous ambush. It may well have been in case of an attack from Bethel because he had recognised more clearly the threat posed by it. What he had probably not realised was that armed men from Bethel had already reinforced Ai (Joshua 8:17). (Troop deployments are often a mystery to readers when viewed without a recognition of all the factors).