There is no number count for back links to get PR but quality is imp, so more quality back links will get you more & early PR
Okay Google just updated its Page Rank the other day and guess what my barely 2 month old blog got a PR3. So there you go, I got a PR3 but I didn't know the answer for how many backlinks needed to get one.
I was raised to set a goal and then work towards it. Then along came the internet and this site is full of "how many links to get pr3, how many links to get to page 1, how many pr2 links to get to ????. Since I started asking these questions I have nothing but respect for any webmaster who wants to know where the goal line is. The internet doesn't work that way! Google doesn't release their trip wires that take you from pr0 to pr3. The same site that is a pr3 today may be a pr0 or pr1 at the next update. So what is a webmaster to do? Easy to say, hard to do: try to accumulate the highest quality back links in a "natural" fashion from quality sites. Try to accumulate backlinks from a variety of sites that are relevant to your site. Seek backlinks on a consistent schedule but develop them with a standard of quality while making your site a place where others want to link. Stop worrying about page rank, number of links as though you think once you reach that "goal" you can stop gaining links. This is a marathon with no finish line, where your competitors are not standing still and thus you can't stop. You can't win a race where the finish line keeps moving, you can only hope to be competitive and if you get to page one it will be a challenge to stay there.
As others said, it's the quality that matters. Get backlinks from relevant sites with high PR. This will help you increase your PR.
I really have phrase "quality links". Everybody says - get quality links - but nobody ever say how to get them.
You are right...actually those, who are telling to get quality links, they know how to get it. And it is called Link Building Service and those who collect links they are Link Builders.