Keyword Density popular question for every beginners!


Everyone suggest me, your keyword should be in your article set with the ratio of 2% of keyword density. While someone suggest me to use your keyword in your article with the ratio of 3%. Which one is correct for me? Second question, while writing a content, is there any limitation to optimize your keywords? Suppose I want to optimize 100 keywords in a particular post with the ratio 2-3%. By the base of 100 keywords, is it possible to keep or improve your PR (page rank)?. Is there any problem to set 100 or above keywords in an article? Please define how much keyword density ratio i should use in 1,2,3> words keyword? Like how, how to, how to download, how to download and install, how to download and install google chrome etc.