PR doesnt depend on number of links. It depends on the quality of links. that is, what is the PR of the page that links to you. also it depends on the number of outgoing links from that page. If it is very high then It will pass very less PR juice. IF the page has less links you get high PR juice. So, you can get PR5 from 2-3 PR6 or PR7 pages if it has less outgoing links. But if they have hundereds of links then you wont get any.
I don't worry to much about pr, build links far and wide and stay with it. Over time your pr will rise.
it depends on the quality of contents and links pointing to it. It also depends on the age of the domain.