All of you people quoting 1-2 per 400 words. where on earth are you pulling these random numbers from? This crap was the norm 6 years ago, 5-10% keyword density etc, but fuck me SEO has moved on by miles and miles.
OP, see what your competitors are doing, use tools such as POP, Surfer, Frase Cora and see what the results there show.
Learn about keyword clustering, semantic LSI keywords, topical relevancy and TEST things for yourself.