Why deterrence depends on more than owning one ship

How can navies reconcile the destructive insanity of war with spending on warships?

The purpose is deterrence: credible, available forces should convince an opponent that war cannot deliver an acceptable victory, and that requires enough hulls and crews to survive routine disruption.

Armed forces provide the lock that makes war appear too costly to be a viable policy. One ship is an intermittent capability, two remain vulnerable to accident or refit, and larger fleets progressively create an at-sea force, a spare, and the personnel depth to sustain both. Deterrence fails when an opponent believes it can achieve its aims at an acceptable cost, so fleet numbers matter as much as nominal possession.