Why warship numbers need depth beyond one ship

Why is building a one-off warship such a risk for a navy?

A single ship can be refitting or unavailable when needed; escort forces need enough hulls to absorb maintenance and deployment gaps.

The Type 82's refit during the Falklands War illustrates the danger of relying on one-off ships or assuming an ally will always be present. The preferred force structure in this answer has nine Type 45s and eighteen Type 23s, giving the Royal Navy a much deeper escort pool. The numbers express a need for enough hulls to absorb maintenance and deployment gaps without losing an indispensable capability.