Why one matching battleship may be an inadequate counter

Could the United States and Japan answer a single unusually powerful British battleship by building one counter each?

One counter would be unreliable because the original ship's navy could choose when to use it. Several matching ships or an agreed limit would address the problem more consistently.

A navy with one exceptionally powerful battleship can choose when to commit it. An opponent with only one matching counter cannot assume that its ship will be ready and in the right place at that moment. Matching a single vessel therefore leaves an operational problem even if the designs are comparable. In this hypothetical arms-control problem, fleets either need several such ships, agree to exclude them, or tolerate an acknowledged exception.