I found a site having with only 10 backlinks and this site gain pr 5 in first google update, how is possible to get 5 pr in first update.
The number of backlinks have nothing to do with the PR of a page. A site can get to PR9 with a single link from a high PR10 page, while at the same time a page can have a very low PR with 1000's of backlinks if they themselves have a low PR.
Backlinks obviously do have something to do with page PR because if you get a link off pr7, most likley will get Pr5-6. It depends if there quality links or not..