Very good question,You can have three to four keywords for a page and make sure about the priority,competition and relevancy of the keywords with page.
It depends on your website that how many pages in your website. But you can target 2 to 3 keywords per page.
You have the answer dude... Agree with this... You can target 2 to 3 keywords per page but try to use long tailed keywords that have a high search volume but less competition.. Good Luck..
I have some sites, with a content stuffed with keywords more then 2 or 3 keywords which actually affects the rankings but, still that sites are ranking like on 30th position or in 50s.
I dont know what they do but, that sites are not worth.