How many incoming links i need for getting PR5?

Hi, How many incoming links i need for getting PR5?
What is importance of free directory submission to get higher PR? Is there a number that i have to reach in terms of backlinks to achieve PR5 ?
Would exchanging links with a few quality PR6-7 links help ?

Basically page rank is based on your website traffics, domain age and quality and quantity of back links . so you can increase the back links and website traffics and its can be improve your page ranks.