6M units would require a rise.
Assuming something like 1M units, which I think is leaning towards optimistic, for the remainder of CY14, the system would need to do the same next holiday quarter (CY15), while averaging something like 135K for the other 9 months of the year, to reach 6M. That's Mario Kart release level sales consistently.
Even 5.5M would likely require the system outperform this CY consistently.
sales are at around 2.85m right? anyway the wii u sold 754k from oct-dec and 1.217m for 2013. if the platform continues its 50-50% yoy growth, that's another 1.1m this year for about 1.82-1.88m ytd. october should offer a better look at the system's holiday prospects, as september's sales were slightly down, offsetting the price drop from last year with the new bundles.
anyway, if the wii u does hit that number this year, that puts the ltd at around 3.95m. it means that for 2015, the platform would actually need to sell 1.6m for the year to reach the range i set. that would be steady/a slight decline on 2014, reachable through constant game releases and probably a price drop as well.