How larger battleship guns reduce the practical battery

How many 18-inch guns could a King George V-sized battleship carry?

Roughly six in three twin turrets, compared with nine 16-inch guns or twelve 14-inch guns in the same broad progression.

This is a rough design heuristic rather than an exact engineering formula. A scheme built around three quadruple 14-inch turrets might become three triple 16-inch turrets when the calibre rises. Moving again to 18-inch weapons could reduce the battery to six guns in three twins. The immediately preceding answer adds that retaining three quadruple 14-inch turrets in the King George V design would save too little weight elsewhere; the ship was already at the limit, so its declared tonnage would require a major fudge. A larger weapon cannot simply replace a smaller one barrel for barrel within the same design envelope.