Without knowing anything at all more or less I'm gonna say somewhere around 30-40m, and this is my breakdown year by year; 1 year, 5m; 2nd year, 7m; 3rd year, 10m, 4th year, 7m; 5th year, 5m. That's 34 million units. Might be slighty more than that. Maybe 40m like I said above. I guess it all depends on the price, the games and the specs.
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Furthermore, I think VGChartz should add a "Like"-button.