I don't know or there is a maximum allowed on keywords. But when you put them too much in your site it will look spammy. I normally try to put my keyword between the 1-2% of the text on my site. (When you, for instance, submit your article to ezine it is not allowed to go over the 2% with your keywords)
guys I don't know if this keyword density theory is right , I check many adult sites which appear in the first page of google search for that keyword and they have 11% density on that keyword too how do you explain that
There is no such thing as a maximum number of keywords to rank for. You can have keywords on your title, headers, url, content, and backlink anchor texts. Its completely possible to rank for hundreds of keywords. Ranking #1 for all those keywords is another thing