You can't go by just the PR of the site linking to you - it depends on how many outgoing links are on the page. A page with only 1 outgoing link passes 10x the PR as a page with 10 outgoing links. PR is determined 100% by incoming links to your site.
Most of the free directories pass little or no PR. You aren't going to find a free directory that will put your link on a PR6 or PR7 page. You're going to need more than free directory links to get a PR5.
If you get lots of high PR external links pointing to your site then your PR will definitely increase.
Get more backlink from high PR site, get one backlink from PR9 site can help you to reach PR5 from PR2
There is no way to predict how to get a PR5. You can purchase many links from high PR sites but many other factors come into play. What if the site you link from gets a penalty for selling links? What if they lose PR themselves? What if Google just discounts a link that you paid good money for? The key is to build and keep building solid back links whenever you can. Try to find the ones that could potentially bring you traffic. PR will fall into place as you build good links but you will often fail if you only buy for PR.
since you have PR2 now you need to work more hard to get PR5 in the next update, get more relevant backlink from high PR site. Good luck
best advice would be just to keep building high PR backlinks and see how it goes. there is no sure way to calculate to see how many backlinks with what PR you would need. there are just too many variables come into play as well. like number of outgoing links the page linking to you has, when did google index the page with your link and when did google give you credit for the link, when was the last update and when is the next update, whether its a good neighbourhood or not, and many more... just keep building backlinks. but if you must know how many PR backlinks you would need, then you can use this calculator
Try to get some good PR backlinks from relevant sites. Focus on quality of backlinks instead of quantity.