Are you confused regarding keyword density for search engines? Are you fed up with all the conflicting advice that everyone gives you?
Believe None of It
Keyword density is dead.
Writing for search engines is dead.
Search engines make their money by giving people who search something useful. Searchers are sick and tired of reading drivel that is stuffed with keywords. They want anything they find online to read well. Search engines like Google want to keep customers happy, so they have developed systems that check for the “naturalness” of every article and page online.
Ideally your article should contain the keyword in the title.
If the keyword phrase you want to rank in SERPS (Search Engine Results Pages) for is iPhone 6 then your title could be “Why Everyone Needs an iPhone 6”.
Longer keyword phrases should only appear once near the beginning and once near the end of an article.
If your keyword phrase is iPhone 6 benefits over iPhone 5 then use this phrase in your first and possibly your final paragraphs only. Anything more reads unnaturally and you will find your article penalised in SERPS because it is keyword stuffed.
Read through whatever you have written to check that it reads well and as though written for people. It must be written for people because you want people to read it, to tweet it and share it with their friends.
SERPS now depend on the natural links that your work gets from people who like it and link to it because they want to recommend it to their own readers. These are the only links worth having and your content must be great and linkworthy to earn them. If I link to an article I am putting my own reputation on the line by recommending it to my readers, I think long and hard before I do that. Any hint of unnatural keyword density and I will not even read it, let alone link to it.