Why early steam warships often had two shafts

Why did early steam warships often use only two shafts, while later battleships used more?

Shaft count was constrained by hull space, ship size, and reliable power per shaft; adding shafts could raise speed but often demanded a larger or altered hull.

Shaft count depends on available hull volume, ship size, and how much horsepower each shaft can carry without excessive maintenance. More shafts can distribute machinery power and improve attainable speed. Applied to the Nelson class, a third shaft carrying comparable power might raise Nelson or Rodney to about 28 knots. A four-shaft arrangement might approach 30 knots, although that estimate is a design counterfactual rather than the historical performance of either ship.