Hard to say, depends on a few things, supply in North America being chief among them.
It should be able to sell about 1.5-2 million by 2013 with 70% or more being in North America, Japan is hopeless for MS and Europe will have issues with so many different cultures, the DRM and Kinect and always online part (some countries have very strict laws regarding this), many US and UK centered games, lack of full functionality on the media capabilities that are mainly a US focus from launch (again) and the price being high.
I think the One will do very well in North America, terribly in Japan and okay in Europe, the UK will likely go bonkers over it though, with them being a "mini-US" in consumer electronics and games.
I do maintain that the 8th gen will see less total hardware sales than the 7th gen though, I see no reason to think why not with the markets diverging and consoles becoming more PC-like and other devices offering full media capability and connectivity.