Back in the wee early days of SEO when SEOs were far more obsessed with topicality than they are now (because less was known about it, and because it was possible to rank on topicality alone) there was a lot of speculation about what exactly the ideal keyword density was.
The idea here is that if you used your keyword in the content more often, you would be more topically relevant. But, if you used it too much it would look like spam. SEOs guessed that Google detected that spam by setting a threshold for keyword density: perhaps it was 5%, meaning 5% of your words should be your keyword.
It probably goes without saying, but this led to some truly awful content that read like it was written by a used car salesman in the late stages of syphilitic dementia.
Keyword Density: It’s Not a Thing
Pretty quickly, however, we learned that in fact Google had no keyword density formula. It turns out Google’s algorithm had much more sophisticated ways of determining if you were spamming, and in fact a lot of those people trying to follow keyword density formulas did poorly because their content still read like spam.
Don’t worry about exactly how many times you should use the keyword in your text. Write naturally and make sure you mention the keyword or variants several times, and at least once every couple of paragraphs, and you should be fine.
Keyword density is good for one thing though: along with the meta keywords tag, it’s a sure-fire way to identify people who don’t actually know SEO, but claim they do.