Two ships normally give a navy enough evidence to separate a design problem from a one-off human or mechanical failure. Dreadnought was an exception because her hull form, machinery, and other concepts had already been explored in preceding ships such as Duke of Edinburgh and Black Prince. The new battleship assembled those established elements into one coherent design rather than testing every principle from scratch. This distinction explains why an experimental class may need sisters while a demonstrator can stand alone.
Notable Point · 31:56-32:33
Why Dreadnought was built as a single ship
Dreadnought was a one-ship demonstration because the Royal Navy had already tested its component ideas in earlier designs.