Each page that you're targeting should have a central focus keyword.
This keyword is not something that you just repeat a "magical number of times" for you to rank.
No!
The target keyword of a page should inform you and guide you as to what information to put on that page.
Having a keyword target is not a license to keep repeating that keyword.
Instead, it's an invitation to deliver value.
You have to understand the intent behind that keyword.
Deliver on that intent, and Google will like you more.
Miss this and just "snipe" keywords, and you're not going to get the result that you were hoping for, assuming you get any results at all.
It's also helpful to optimize your content.
So you have a target keyword, plug it into tools like Postpace or Surfer SEO, and you will get a list of all these other related keywords.
Now keep in mind that these tools didn't pull these keywords right out of their assholes.
Instead, they studied your existing competitors and paid attention to the keywords that they mentioned.
So whatever contextual keywords that you are seeing are based on real-time competitor analysis.
Ignore them at your own risk.
Once you've used those tools and you are focused on keyword intent, then you have all you need to craft content that can rank.
You have to stop thinking in terms of how many keywords I can use on a page.
Instead, focus on targeting the right keyword, delivering on the intent behind it, and making sure you can meaningfully discuss contextual keywords related to it.