Well, there is no optimum keyword density to rank higher in SERP’s. Google suggests writing natural articles with your targeted keyword in the article. The best way is to have 1-3 % of keyword density along with Semantic keywords and LSI keywords. This way you will rank higher in SERP’s. Do not overdo and repeat the keywords multiple times unnecessarily, google calls it as keyword stuffing and punishes if it’s overdone.
The ideal keyword density for any piece of content is considered to be 3%. But , the content should not looked as stuffed.
To know more about the SEO mistakes you can always read this article as it is well on the point.
5 Common SEO mistakes to avoid while doing seo