Why larger guns reduced barrel count

Why did battleship batteries shrink from twelve smaller guns to nine larger guns?

Heavier shells preserved broadside weight, while their ammunition and storage demands made fewer gun positions more practical.

Total broadside weight mattered more than barrel count by itself when capital ships moved from twelve smaller main guns to nine larger weapons. Larger shells can maintain or increase the weight delivered in a salvo even when fewer guns are mounted. Their greater storage demands also limit how many rounds and magazines a ship can carry. The heavier ammunition therefore constrains the practical number of gun positions.