Why twelve destroyers is a precarious global minimum

What is the minimum number of destroyers needed to protect a fleet and trade routes?

It depends on the navy and missions, but a globally active navy falls into difficulty below about twelve because maintenance and readiness cycles may leave only four operational ships.

A nominal force of twelve can be organized into three groups of four. Under a demanding availability cycle, about six might be broadly available and only four operational. A carrier group could consume two, leaving only two for other tasks such as trade-route or chokepoint presence. This arithmetic explains why the Royal Navy originally sought twelve Type 45s and why equipment flexibility cannot substitute completely for hull numbers.