Actually looking at it again, and I had to estimate a bit because the fiscal year Nintendo uses doesn't allign perfectly with a full calendar year of Jan-Dec, but even when I didn't include the final quarter for both 3DS and Wii U, this is what they basically shake out as:
3DS shipped approximately 15 million units in its first 12-13 months.
Wii U shipped approximately 5.86 million units in its first 12-13 months.
The if NX is supposed to be a line of portable + home console that plays the same games ... then 20 million part orders for year 1 isn't actually unreasonable.
It would basically be the same as the 3DS + Wii U first years combined.