RobH_en_tn/2sa/24/02.md

Dan to Beersheba

This phrase uses two place names Dan, in the far north, and Beersheba, in the far south, to represent the entire country. (See: rc://en/ta/man/translate/figs-merism)

count all the people ... fit for battle

This means to count all the men except those men who are either too young, too old, or physically unable to fight.