How many men would a city of 12,000 have had available for war?
Joshua 8:25 (ESV)
25And all who fell that day, both men and women, were 12,000, all the people of Ai.
If the entire population of Ai was 12,000 people, then the maximum size of their army (if only men above a certain age are included) would probably be around 3,000.
25 And all who fell that day, both men and women, were 12,000, all the people of Ai.