Depends on what kind of page it is. For example: Review pages must include just concentrate on one main keyword. If you're going after long tail keywords, be sure there is no place for other keywords except what you're targeting. A page about men/women health can obviously contain several keywords so it all depends on what you wanna that page to do. are you using it just for backlinking purpose and things like that. Without these information it's not possible to suggest any concrete rule.
If you think about it, after removing the non-counting "filler" words (such as a, the, of, is, etc.), every word on your page is a "keyword". The search engines can't read our minds and determine whether or not we are targeting certain words as "keywords". They just spider the page and make note of the countable words that they find. For example, in the first sentence above, the "keywords" are think, removing, noncounting, filler, words, page, and a few others. So it all comes back to CONTENT. Write pages about things you are passionate about, things you are knowledgeable about, things that will interest others. As you look over your rough draft, you will notice certain "key" words and ideas standing out from the overall copy. Pursue those with a good keyword research tool (e.g., Google Adwords), look for good synonyms, add other forms of those words in ways that still read well. And you will do just fine.
for me it depends on how many words there is in the webpage. if it only has 250 words then concentrated on just one keyword, if 500words then 2-3 keywords is fine. and so on.
You can Put 2-3 Keywords per page, not more than 3 keywords because if there more than 3 keywords th each keyword contribution is low if 2 keywords each keyword 50% contribution if 3 keywords 35% per each keyword so please use maximum 3 keywords per page and use each keyword maximum 3 times in the content not more than 3 times.
Depends on keywords. Some keywords help each other. For example, "buy car online" helps to the keyword "buy car"