As much as possible back-links you can create for your website is better. But I personally prefer that each back-link should have quality and also from relevant website. Because Google and also other search engines highly prefer relevancy.
i dont think and i never read anywhere that these many link needed to achieve these much PR. i think rather than counting links we should focus to get quality backlink for the website which help to get good SERP and thus good traffic which is our ultimate goal
This is really funny. Well count of back links depends on the competition and keywords for which optimization is required. There is no set of rules or any standard amount described for building back links.