I contacted them and was told that at this time, there is not a limit. However, I did contact them when YPN was first released and I was invited to join. Their terms might have changed by now. But I am still running 3 ad units on some pages.
just to be crystal clear: right now we are not very smart about duplicate ad layouts on a single page and thus you are very likely to get duplicate ads if you do this. however it won't break anything. if you have a big page with alot of content then putting another ad unit at the bottom might be helpful even though the ads are typically the same.