No one can say HOW many links you will need to get to a PR.
What it really depends on is how GOOD are the links in which are pointing back to your website.
You just need good links.
Search Engines consider Link Building to be one of the most influential portions of SEO success. Your back link portfolio is the entire scope of websites that are currently linking to your site directly, this does not include link exchanges.You can get Link building service at http://www.prominere.
You can also use Search Engine Visibility to increase your Page Rank You can get this SEO service from http://www.tucktail.com/search-engine/seo-services2.aspx?prog_id=447401&pl_id=447401
Here they provide you One-click submission of your site in many search engines like Google, Yahoo!, Bing, and AOL etc... and they also provide Powerful reports that show you how to move your website up in search results.