I don't really think there's any way to predict sales unless we have some way to predict how many units they can produce period if the systems became widely available tomorrow, and say that way through the end of the year, then I can see 10 million or more in sales. If the PS5 stays supply constrained but Xbox does not, then sales will go even higher. But, if they don't get supply straightened out until april, or they have other supply issues throughout the year, they obviously cannot sell systems that they cannot produce. So, in that case I would predict that they sell every system they can manufacture.
while it does of course make sense to compare to their previous system, I think the current generation strategy is much better than the last. They now have systems at two price points, giving them both the most powerful and the least expensive console this generation. They also haven't done as much stuff to piss gamers off this time around. Add to that game pass subscriptions that will limit the number of people that might switch to PlayStation this generation, and you have several factors in xbox's favor. So, I expect sales to be better than Xbox One throughout the generation. Again, that's assuming that supply issues and soon.