‘ And all that fell that day, both of men and women, were twelve eleph, even all the men of Ai.'
This figure probably included the men from Bethel. The habit of mentioning the allies only once and then assuming their presence occurs elsewhere. Compare Judges 3:13 in a passage which might give the impression that only Moabites were involved, while the Ammonites and Amalekites were involved too; and Judges 6-8 where it is the Midianites who are prominent in most of the passages even though they have allies in the Amalekites and the children of the East. Consider also Judges 12 where Ammon clearly included Moab (the king of Ammon uses Moabite claims as his basis for his demands) even though they were not mentioned.
But, as we have suggested earlier, it may well be in this case that the men of Ai and the men of Bethel were in fact one, with the original men of Ai being a vanguard for all Bethel. All had in Israel's sight issued forth from Ai. Thus the men of Bethel were men of Ai.