In general, console sales during the holiday season tend to be equal to the sales of the rest of the year combined, usually within 20 percent.
According to VGChartz, the Wii U has sold 1.25 million units so far this year. Assuming the upper-end of that estimation...
2.2 (220%) * 1.25 + 2.25 (2012 total) = 5 million.
However, given the circumstances surrounding the Wii U sales (namely, the game drought), and the recent price-drop (which hasn't been observed long enough to guage its effect), that's probably too conservative. Without more data, I'm not comfortable shooting that low.
In the end, I think the highest the Wii U could sell before the end of the year is 6.25 million, LTD.
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