During their last results release, Nintendo claimed they had shipped 13.2 million Wii-U units and planned to sell 800k in the next financial year. By simple addition and the fact NX is out next year, it seems to me like Nintendo only built 14 million consoles. It's likely they stopped producing them during the (disastrous) Year of Luigi.
From my observations though there's no need to run out and grab a Wii U. Gaming shops are piled high with preowned consoles as well as loads of boxed new consoles. With the sparse software release schedule (2-3 more games for 12 months) expect that number of preowned consoles to increase.
I think also one can wonder if Nintendo are looking into doing an Atari and burying all the unsold consoles and games in the New Mexico desert. They've already copied Atari by angering their fans with low quality, unfinished games (Pac Man, Star Fox Zero) and insisting it's the gamers at fault and not their hardware designs (Jaguar, Wii-U).
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