I know this question sounds sort of stupid since quality of BLs are also very important. But let's assume most BLs are low quality (PR=0 or 1) since I'm unlikely to get high quality BLs. So far my site has a PR 1 with about 150 BLs.
I want to know approximately how many more BLs i need to increase my site's PR?

There is no particular answer for this question, :)
However, if you want to get more pr, you have to get backlinks from your niche related sites, as well as links from the web pages which have good PR, less outbound links etc.

I had an experience, I got only 5 links from PR4+ pages (few are not related to my niche), That brought me PR4 from PR0 :D. So, it depends.

Hi,

There is no thumb rule that you need particular number of backlinks to get specific PR.

No one can tell you the exact number, just focus on link building and keep your self away from "nofollow" tag.

The page rank assigned by Google depends on the number of back links not only that but also depends on the quality of back links. You cannot guess a fix number links as the quality of the links also matters a lot. I agree to all who has posted their reply as we cannot tell the exact number of links.

its depend on ur backlink quality

I think it doesn't matter how much, but from where, if you get 30 PR 7 and up back links your PR will fly!! :D :)

It depends on the QUALITY of inbound links that you are getting. The more quality of inbound links, the higher chances for your site to get high PR.

more it depends on content and quality of your links~

In my opinion you will need about 500-1000 backlinks from PR0 sites for a PR2 ranking, But it is just my opinion with some experience.Theme based backlinks help you to get PR 2.

there is no such rule and principle
quality is much more important than quantity

I heard that around 2000 required to get page rank and so on....

This is totally depend on the search engine like Google.

Its not based on the number of inbound links but the quality that you are getting. Google gives more value on the quality so create links from relevant and DoFollow sites.

PR is based on the PR of the pages that link to you. So it is not a matter of quantity, it is more about quality. Your DP signature links probably won't pass any PR to your sites. The pages with your links are likely all PR0.

PR doesn't really matter though. Focus on your rankings and traffic more.

there is no specific numbers. but do backlinks continuously and remember they should be of good quality.cos only prefer quality links.

As far as the algorithm of Google is secret, we cannot predicate number of backlinks required to reach PR2+, However, if you can keep on getting your niche related backlinks as well as if you update content often, you will get good Pr.

If i says that you need 500 BL from PR3+ it wont be wrong, I am saying on some assumption which i read on top SEO site where they were telling about how many links you need to get on certain PR. The Algo is kinda mix with greater PR as you have described it also.