Every extra anti-aircraft gun needs people, space, and ammunition

Is a very heavy battleship anti-aircraft battery practical simply because deck space is available?

Only if the ship can also house and feed the gun crews, store enough ammunition, and sustain the weapon without creating a separate supply burden.

The design problem extends from the mounting itself to crew accommodation, food, water, sleeping space, magazines, and ammunition endurance. A gun that empties its ready supply immediately is not useful, while an accompanying ammunition ship creates another vessel that must be protected. Fitting more weapons is never just a deck-plan exercise.