NSMB is ridiculous expectations. That game is going to sell 6 million in Japan and maybe 14 million worldwide--possibly more, its legs could hold into a 3rd and 4th year... It is Mario, afterall.
5 million sales WW would be a big success for PH. Only 2 Zelda games have ever sold that much (OoT at 7.6, LoZ at 6.5), and Twilight Princess combined sales will make 3.
And in Japan, no Zelda game has ever broken 2 million. I'd say that's a realistic (if bullish) goal for PH, given the popularity of DS.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.