It really depends on your SEO strategy... For me you must have to classify your targeted keywords into 3 aspects such as High prio, Mid Prio and low Priority keywords... In this way you can set your Link building campaign to be organize.
5 - 10 if relevance with your site... try using keyword ad word... <-- the name to long i not remember that
You can target as many as keywords you want per site. However, I would advise you to target only one to two keyword phrase per page
Too much keyword cant targeted. So 1 or 2 is enough after u get good position many supporting keyword will automatically come up from your site content.
Keywords are something that is on going and you should try to optimize for many different tails of the same keywords.
If your site is quite new, better to target less than 20 keywords so that you can focus more in increasing their rankings. You can divide them into two: (major and minor keywords)
It depends on your websites. I target minimum 3 keywords per page. if your site has many targeted keywords or you are confused for number of keywords then you can set keywords as High priority and low priority.