you need to use your site specific keywords on specific pages for proper optimization of those pages, there can be hundreds of keywords specific to your site and used appropriately on different pages
There's actually no limit. Like Copymoves said, they should be relevant to your site's content. But, more the keywords, more the hard work needed to rank them.
I wont target more then 3. Each keyword we target we should have proper keyword density and other SEO factors reflect them.
In the meta keywords? In the meta description? In the titles of articles? In the alt / title text of a link? In the first sentence of the opening paragraph? Keywords in backlinks? Could you be a little more exact please? My main forum targets 3 phrases, and each phrase contains 2 keywords. But, google ranks my main forum in the #1 spot for about 45 different search phrases. Most of them are not my targeted keywords.
the recommended is for the major 2 or 3 keywords or 5 or 10 is OK for minor keywords. however there is no strict rule. it depends on the content of you site.
Infinity, if you have that much of pages on your site. so select your segment, and try t get best keyword which is easily can be ranked in search engine or can give you profit on your business.
I think if your site is new then first target with one keyword, if you get rank for that keyword then try to optimize for other relevant keyword.
There is a very fine line between LSI & keyword stuffing. As long as you are not crossing the line and end up diluting the theme of the page, target as many terms on a page. Mostly the theme breaks and get diluted on any thing above 4 keywords. So the bottom line is don't be greedy and endup confusing the search engines and be very precise what you want to rank for.
I guess if it's your main page then it shouldn't be too many, maybe 20. But your other pages should only target 1 or 2 really.
There is no answer to this question. You can adopt for a general site targeting multiple keywords or a very niche site targeting just a few, the key is how you promote the site, not how many keywords is has.