Question : SEO: pros and cons of different number of keywords per page

What are the pros and cons of optimizing 1 vs 3 vs 5 keywords per page?

Answer : SEO: pros and cons of different number of keywords per page

A keyword is only a keyword for a specific page because of its prominence in relation to that specific page.  The more prominent it is, the better "optimized" you can say that page is for that keyword.  This is essentially what the search engines measure - the prominence of a keyword in relation to individual web pages.  In the case of Google, there are over 100 factors in determining how pages rank for any given keyword.  At the moment, two very important factors are the presence of those keywords in page titles and in the anchor text of that page's backlinks.  Over time, some factors become less relevant (such as keyword density) or irrelevant (keyword meta tag), while others become more relevant (such as linearization).

The more keywords you optimize a page for, the less prominent each individual keyword will be.  This become especially true when you have too many keywords to fit naturally and effectively into your page titles, anchor text, etc.  If you target five key phrases, you may only be able to fit one or two into your page title (if you are squeezing in three or more, you may want to reconsider).  The same goes for your anchor text.  If your page has 100 backlinks, you are much better off if all 100 links use all of your targeted keywords in their anchor text (in varied forms, of course) than you are if 50 of them use two keywords and 50 use two different keywords.