One may use loads of keywords per page but that doesn't do any good in terms of SEO. For a better On page optimization, the number of keywords targeted per page should be in a limit. According to me the number of major keywords should not cross more than 3. However there can be long tail keywords which do not actually have any limits as such. Give in your views on this!
I agree about the number of main keyword should not be more than 5 - h1, h2, and 3 in the paragraphs.. But the more important thing is keyword density.. right now... the best density is around 0.5-1.5 % (at least for my site, when i lower the keyword density buy subtituting with relevant/LSI keywords, the rank improved a little bit)
Including keyword in the meta tags/ headlines is necessary and effective. As I heard about that the best keyword destiny is 3% to 5%, which is actually difficult to reach. I'm not sure whether that number is still valid after Google penguin updates.
The purpose of SEO is to get a lot of new customers through high rankings on Google and other search engines. If you have taken the time to find the best keywords for your website, you probably have a very long list with many keywords.
For me, what matters most is, your visitors will enjoy the content that you posted on your website. Personally, I think just 1 to 2% of keyword density is fine. Don't over-optimize your website.
I've understood that you are asking for the number of major keywords that can be used per page. I'd say, don't exceed 3 to 4 because this would again cause lots of unbalance in On page optimization. Also try to maintain a keyword density of around 2%.
I'd say keyword density should be around 1 - 2% nowadays, give or take a 1/2 %. My suggestion is to do it and then move on to other ways of getting traffic so that you are not focused on just SEO, or one thing (not that you are, jmo) and let all your traffic sources work well together.....