YPN Blog states:
We’ve always maintained that artfully interweaving our ads with your original, relevant, entertaining content is a common sense best-practice. This helps ensure quality leads to our advertisers while helping you maintain your publishing credibility. Over the long term, enhanced credibility with users may lead to a wider audience and more frequent return visitors.
To encourage this best practice we have updated our Program Policies, which now require that no more than three ad units appear on a single page at one time.
Earlier, no specified limitation was imposed on the number of YPN ads one can place on a page.