According to Moses, this verse is counting all the young men from the tribe of Benjamin who are 20 years old or older and strong enough to be soldiers. It's like a big list of all the people who can join the army from that family.
Numbers 1:36: Of the children of Benjamin, by their generations, after their families, by the house of their fathers, according to the number of the names, from twenty years old and upward, all that were able to go forth to war;
In this verse, the number of people counted in the census is different from the number in Numbers 1:36.
1 Chronicles 21:5: And Joab gave the sum of the number of the people unto David. And all [they of] Israel were a thousand thousand and an hundred thousand men that drew sword: and Judah [was] four hundred threescore and ten thousand men that drew sword.