Google’s bots and algorithm look at the keyword density in your copy differently than other search engines. Yahoo, for example, likes a very dense copy, where as Google is a bit leary of finding a keyword too often.
In general, 3-5 times is enough to mention your keyword(s) in the copy of a given page of content (500-100 words). Make sure the placement is smart, in a strong context near second-tier keywords and the like, that it is at least once in the title of the page as well as in the heading h1 tags.
Once you start to repeat the keyword more than 2-4%, Google smells spam. It will distance itself from your site and lower the ranking, simply because it has suspicions that your content is spammy, overtly targeting the keywords rather than being a natural rich source of content related to the keywords.
Well, Google is right on the money of course (at least as far as trying to target the keywords).
Just remember to take it easy and focus more on the value of the content to the reader than the richness of keyword placement. 3-5 times, and your fine.
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