As far as I can tell, Yahoo has not set a limit on how many ad units you can put on a page.

But the problem I have seen - is that the same ads are served over and over again in separate units (the same ads shown in Unit 1 are also show in Unit 2).

So what's the point of putting more than 2 YPN ad units on a page? One at the top, one at the bottom of a page.