How many keywords you should target per webpage ?

Target different keywords for different pages. Here pages means inner pages or inner URLs.
Never target single word keywords and target search phrases.
Target one or two main keywords per page. That is don't target many highly competitive good keywords for a page.
Beside primary or main keywords you can target 3 to 4 not so important keywords for a particular page.

To know more about Keyword Analysis visit this link : http://www.rankingoogle.com/keyword_analysis.html

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