USN ships are divided into Departments. Navigation, Operations, Gunnery, Supply, Disbursing, Engineering is the old "six department" split. These were also known as Navo, Oppo, Gunno, Suppo, Disbo, and Snipes from the shorthand names for the department heads.
Departments are then subdivided into Divisions, the number of divisions needed relating to the size of the crew, the size of the ship, and so forth.
As a guess, only the 20mm or 40mm gun mounts need at least 11 divisions; the 40s could be too, though, by personnel count, since it's 10 per quad, versus two per Oerlikon. 20 quad 40s is 200 personnel plus the gun directors; 49 20mm is 98 hands, plus runners minding the ready-service lockers.