My personal opinion is to write an article that is useful and look natural. However it is good to check how top 10 competitors have organize the article and how many times keywords that you target will appear in the text. The most important part of writing an SEO article is to choose title properly. As well using H2,H3 tags and correct punctuation.
One more thing, it is also important to choose the length of article which can vary between 300 to 500 words.