The opinion on this subject are very different, some say that the perfect amount is about 3%, some say that even 10% is good and others believe that an amount of 2% is the optimum quantity for keyword density. If you take a look at all the articles on the web, there’s a bunch of gibberish which doesn’t give you a clear idea on how many keywords you should use. I don’t know any big secret either, but I’ve noticed that good combination of keywords and content must result in finding those keyword in the first 100 words of your article/content.
Since 2007, Google declared that keywords are pretty useless, since anyone can use any keyword, not related to the actual content. This measure seems pretty fair to me, if I want to get results for a website selling shoes, I don’t want to be outranked by a website that sells sex toys but uses my keywords. It all sums up to the content! Still, a lot of us are using keywords, either because we’re just use to them or we still believe that they make a difference.
So, to answer your question, I think 2% keyword density is enough, that is if you strongly believe that today’s search engines algorithms are so dumb to take this into account. The keyword density becomes a myth with each improvement of search algorithms, but hey, who am I to tell you what to do? You stick to your methods and don’t forget to deliver good content with a high relevance.