The honest count is “as many as one intent naturally contains,” which means the number follows the search, not a quota you set in advance. A page should cover the variations and sub-questions a single searcher would treat as the same query, and that total is fixed by the topic, not by a target you choose before writing. You will see working figures quoted, one primary keyword plus a few supporting ones, often three to five, and that range is a fine sanity check. But treat it as a guide to validate against the actual search, not a rule: a tightly bounded intent might hold only two or three supporting phrasings, while a broader one can naturally rank for dozens once it is covered well.
The way to find the real number is to read the intent, not count slots. Look at what the top-ranking pages for your term already cover, and at the questions in People Also Ask, and let those define the sub-questions your page owes. Each genuine sub-question inside the intent is a supporting keyword worth covering; a phrase that sits outside the intent is not, however related it looks. Cover the intent fully and the supporting terms show up on their own, because you wrote about the things searchers actually wanted rather than because you hit a figure.
So the count is a result, not an input. List the sub-questions that live inside your one intent, drawn from the results page and from People Also Ask, and cover those, instead of trying to hit a fixed quota of supporting keywords.