Relevance is the question people skip because it feels obvious, and it is where most keyword plans go wrong. A query is relevant to a page when somebody typing it would be satisfied by what that page actually does. That is a stricter test than sharing words, and it is a looser one than being a synonym, which is why lists built by matching words produce pages that rank for nothing and lists built by intuition miss half the traffic.
The test worth applying
Read the query, then ask what a person typing it wants to happen next. If the answer is what your page does, it is relevant. If the answer is something adjacent that your page mentions, it is not: mentioning is not answering, and a page that mentions six things answers none of them well. Applying this honestly usually removes a third of a list, and the removed rows are the ones that would have wasted the writing.
Secondary keywords are the same intent said differently
A secondary keyword earns its place when somebody typing it would be happy with the page you already planned. Different words, same want. Those belong in the same page, and the page should use the words people actually use rather than one canonical form. What does not belong is a different want that happens to share vocabulary, which is a second page pretending to be a phrase.
How many is too many
There is no magic number, but there is a symptom. When you cannot write a single opening paragraph that would satisfy every query on the list, the list is at least two pages. That test is more reliable than any count, and it catches the common failure where a page tries to serve both a beginner definition and a buying decision and reads as neither.
Relevance to the site, not just the page
A query can suit a page perfectly and still be wrong for you, because the site has no standing on that subject and no reason to be believed about it. Pages that stray from what the rest of the site is about tend to underperform their difficulty score, and the honest response is usually to leave the query to somebody else.
Questions people ask about relevant keywords
What makes a keyword relevant?
That a person typing it would be satisfied by what the page actually does, not merely that the page contains the words.
What is a secondary keyword?
A different phrasing of the same want, which the page can answer without changing shape. Serve it on the same page, in the words people use.
Should I put several keywords in one page?
Only where one opening paragraph would satisfy all of them. If it would not, you have found a second page rather than a secondary keyword.
Does repeating a keyword help?
No. Write for the person; use their words where they are natural. Repetition beyond that reads badly and does nothing for the page.