They MAY be able to do 9 million units in total, 4.5 million units over the next six months would still be a challenge.
Nintendo always get a huge boast in sales during the holiday so 4.5 million in that period is pretty doable. Last year they sold 3 million Wii U during the holiday at $350 and the only major title being NSMB U. Well this holiday the system will be $300 with NSMB U bundled with it, along with the Luigi DLC included. Combined with more major titles like Mario 3D World, I think they'll be able to do at least 5 million during this time.
But this year they've got more competition. That's gotta be a factor. Plus, Nintendo is still having some marketing issues with the Wii U and the XB1 and PS4 are already starting to eat up the mindshare.
I have to agree with Shyguy and Nickmitch.
In a normal year, Nintendo might have met these sales expectations. Nintendo typically pull in great numbers during the holidays. This year, however, both Playstation 4 and Xbox One are expected to sell out at launch, and both companies are reported to have pretty solid production yields. That means there is going to be a bunch of new consoles out there for people to buy, which sucks a not insignificant amount of money out of the market.
I also think Nintendo already played the Mario card when they launched the system with NSMB U, and as a result many of those people who were interested in playing a Mario game already own a Wii U. The packing in of NSMB U, Luigi U, and the bundling of SM3DW will definitely boost sales, but as good as those games might be I don't see them bringing in the numbers that Nintendo need.
Whether or not Nintendo cares to acknowledge it, it is competing with Microsoft and Sony directly for people's dollars/pounds/euros, and the launch of these new consoles just means increased competition.
I of course hope Nintendo meets, and in fact exceeds, these expectations, but I'm dubious.