The Book of Numbers is the fourth book of the Bible. It tells the story of the 40 years which the Israelites spent in the desert before they came to the promised land of Canaan. It also contains a census recounting the 'numbers' of men available to fight in the coming war.
Numbers 1:2-3
"Take ye the sum of all the congregation of the children of Israel, after their families, by the house of their fathers, with the number of their names, every male by their polls; from twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies"