EDIT: thought this was T&C. Don't worry about the density or number of times you use a word. Go get your list of related or "LSI" terms and sprinkle them through the article where appropriate...no need to force things and make it sound silly like what I posted above.
I usually try to get my main keyword somewhere once in the first couple of sentences and once in the last couple of sentences. If it shows up elsewhere in the article writing natural-sounding prose, then great. Along with the use of related terms you should know if you have researched the niche, Google can easily figure out how to categorize your content.