I'd say 5M would be a moderate success. 10M and over would be a true success. Personally, I see it doing ~4M-5M. The biggest thing going against the X, though, is the price. It's $500, but with the same limiting Jaguar cores. I don't think many are going to see a big enough difference in the X over the Pro to spend an extra $150, since I expect Sony to cut the price to $350 this holiday season. Things will get even worse next year. While MS will probably cut the price to $400-$450, Sony will most like have the Pro down to $299, keeping that $100-$150 price difference, while also hitting a mainstream price.
Of course, next year I fully expect we'll start to read rumors of the PS5, which will be coming 2019, or 2020 at the very latest. Those rumors could possibly put people off from buying an X. And in 2019, when MS can finally get its price down to ~$300-$350, we are going to either have the launch of a $399 PS5, that will most likely be at least 2x as powerful as the X, or we will have its announcement.
