55m. Or are you asking how much it sells before the next iteration is out?Edited op to be more clear. You're correct.
6 mil this year, 3 mil next year, maybe a couple mil after that. I doubt Microsoft will try to push Xbox One sales once next gen starts.
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35 million sales from Nov 2013 to January 2018. If you define a year as January to January, then that's (35-3 (MS announced 3 mil sold in 2013))/4=8 million average a year.
MS's last official statement said the Xbox One had almost shipped about 10 million in the first year so 8-9 million sold a year lines up. Also shows that the first year wasn't super low and 2017 was very unlikely to be above average.