In practice
One primary keyword per page, supported by the variants people actually use. Two pages chasing the same keyword compete with each other.
Judge a keyword on intent and value, not volume alone — a term with 200 searches and clear purchase intent beats one with 20,000 and none.
Why it matters
Keywords are how a content plan connects to real demand instead of internal vocabulary.
They are also the unit the whole programme is reported on, so choosing badly distorts every number after it.