Actually, YPN does specifically state this. This may be new and I don't think many people are aware of it. I know that Adsense has this limit, and YPN may not have had it before.
But, now they specifically state that "The Ad page may contain a maximum of three ad units," on their program policy's page.
So if anyone has more than that, then they may be able to get away with it for a while, but sooner or later, they'll be hearing from YPN.
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