There's no fixed formula for how many related terms a page needs — the right amount is whatever genuinely fits the depth and length of a thorough treatment of the topic.
Let the Topic's Natural Scope Decide
A narrow topic naturally supports fewer related terms than a broad, multifaceted one, and forcing extra terms onto a narrow topic tends to feel padded.
Longer Content Can Reasonably Include More
A comprehensive guide has more room to naturally incorporate a wider range of related vocabulary than a short, focused page.
Prioritize Natural Fit Over Any Specific Count
A related term that doesn't genuinely fit the content is better omitted than included just to hit an arbitrary number.
Next step: Use the LSI Tool to generate a related-term list sized appropriately for your specific piece of content.