Note that by adding a blog to your site, you can target substantially more keywords than you otherwise could.
Target keyword are depend on your site and their niche.We can not exactly say how many keyword should be there?If your site related to highly competative niche then you have to choose proper keyword to beat your competitors.
More particularly you need to target its own versions and one keyword for each page of your website. Despite the fact that that's accurate, there are pages which you actually can't optimize for target keywords depending upon your company. Apart from that, you must optimize each page for a phrase. The amount of pages you could practically optimize for target keywords will be the variety of keywords you'll be able to target.
If you are asking how many keywords a website can have, then it does not have any limit. You can optimize as many as keywords for your website as you can't say from typing which keyword,people will reach your website..
I think it depends on the requirement of your page, but it should not overloaded so that it looks over optimized in other words it should not look like that there is a keyword stuffing in the content which may lead to dropping in your website rank.
I think you must prepare the keyword according to your site's services and functionality it can be so many in numbers but when it is about a single web page then bulk of keywords in a single page should be avoided.