There is no magic number when it comes to keyword density. According to Matt Cutts from Google, “It’s not a hard and fast rule.”
Keyword density was a big ranking factor many years ago, but today, the main thing you need to worry about is not using a keyword too much. When you stuff the keyword phrase in everywhere you possibly can, you not only end up with an article that is unpleasant to read, but the search engines start to diminish your ranking for that keyword because it looks like you are trying to manipulate the rankings.
To get your article ranking well in the search engines, you really don’t need to worry about keyword density at all. Just make sure you use your keyword once or twice in the article. Beyond that, write naturally about the topic and most of the time you’ll do just fine.
Google is constantly changing its algorithm to try to put more quality content on top, so if you write quality content, you’ll have a good chance of surviving future updates. Focus on your reader first and foremost, and write what you think they are looking for.
For example, if you’re targeting the keyword phrase “how to tie a tie,” it’s pretty obvious what the searcher is looking for. If you’re serious about ranking for that keyword phrase, you should provide easy-to-follow instructions for tying a tie. Better yet, include photos for each step, or maybe even a video demonstrating how it is done.
If you do a good job on your post, you’ll probably find that it attracts links from other sites naturally, and it will start rising in the SERPs without much more effort from you—regardless of how many times you used the keyword phrase.