The YPN Blog posted that they have updated their program policies to limit the number of ads you can use on a page by page basis. Before, there was no specific limit on the number of Yahoo! Publisher Network ads you can place on a page. Friday, that changes to a limit of three ads per page.
The Ad page may contain a maximum of three ad units.
I am not sure how many real sites had more than three YPN blocks on their pages anyway. So this should not be a huge change for the majority of publishers.