My opinion, no limits, but if backlinks comes from spam bots on forums and blogs and still be ranked not profit, incoming PR6 and PR5 backlink and few PR4 is giving that website huge power. And remember that its not the domain
PR that counts but the page
PR, get SEOquake firefox plug in to see the page
PR.
So how many backlink is enough? All you need to do is get more quality backlinks then that website. More backlinks from PR6-5-4-3-2-1.