no one can ans this ,because google is not disclose this yet
There are no specific numbers of backlinks for get high PR. Its depends on your quantity and quality of back links. You can choose other activities’ like, blog commenting, article submission, social bookmarking, forum posting, guest posting in high PR websites. It’s really helpful for you to getting better PR. Link exchange is also good for you.
There is no exact or definite number of links required getting in a PR5 site classification. I believe that all you need do to is to create high quality links and get more natural traffic so your site becomes more authoritative.