I don't think there is a "set" limit, but use common sense. In my opinion anything more then 4 is overkill. When you have 12 on a page your ads now become your content, which is ridiculous, and I don't think they would be happy
There's not a number limit, it's on a site per site basis. Some sites 4, some sites 2, etc., it just all depends on your site. This is what I was told 2 days ago when one of my sites were being checked by YPN.
A friend of mine has talked with a person from the YPN staff and she suggested no more than 3-4 ads per page.
There's no restriction. There was a poll in the YPN publishers area about how many ads do we display a day.
I wouldn't put more than 3-4 on a page. Usually putting like 1-2 ads on a page performs the best for me.