The more backlinks you have the better your pr will be but it depends on the quality of your backlinks. Backlinks from high pr website will generally give you a more quality backlinks. You can read more about pr at wikipedia if you wish to as there are nice explanation on how pr works.
PR is rather useless by itself, but nonetheless, if you want to increase it, get backlinks from high PR sites related to your own website. I'm an avid fan of placing my signature in any forum related to my website and then answering questions. It keeps the dullness fo SEO work down while accomplishing your goals.