It is not about how much backlinks... It's about how much backlinks from authority sites. A page with 1000 backlinks could rank better than a page with 10000 backlinks. Anyway, are you looking for a high PR or are you looking for good rankings ? Because High PR doesn't mean that your ranking is good.
it is an exponential scale meaning that something like 1000 PR1 = 1PR2, 1000 PR2 = 1 PR3 and so on ... nobody knows really if the multiple is 100, 1k, 10k ... but it works like that. Conclusion: fight/pay 1 backlink with a high PR is the most time efficient method to get PR a high SERP rankings
It is not about the quantity rather it is all about quality. Your links should have quality. If you have quality links your PR will boost in no time.