The inner guns of the wing turrets could not safely fire forward or aft because their blast would damage the ship. The central superstructure also left almost no worthwhile arc for turning either wing turret across the deck. In practice, this narrowed the useful battery to six guns on a broadside. The Invincible example shows why operational firing arcs matter more than a simple count of mounted guns.
Chapter · 8:56-10:16
The Invincible class's practical six-gun broadside
Superstructure and blast limits made the Invincible class's cross-deck wing-turret fire impractical, leaving a dependable six-gun broadside.