You don't have a limit for it ! You can get as many backlinks as possible. Google PR doesn't depend only on backlinks.... Do you have hellboundbloggers.com which has lots of backlinks but still has PR 0 and new blogs have PR 3 +
It depends on the QUALITY of the backlinks. If you get links from HIGH PR sites then you'll be able to gain more PR quickly, if you get links from 0 PR or PR1 sites then it ain't going to get you anywhere soon.