40-65 million. A good chance it will perform worse than Xbox ONE. Contrary to Phil's optimism, I expect the Series S to fall into irrelevance rather quickly, that I'm willing to bet MS and 3rd parties will stop fully supporting it halfway through the generation when tensor-core like technologies become mainstream.
Why would they stop supporting Series S? They likely already sold millions of it and it leads to them having the cheapest 'next gen' console on the market which makes it an enticing option for parents, people who mostly play the yearly FIFA/Madden/COD and everyone with a tight budget. Also there is practically no technical reason to not continue scaling down the resolution down a bit and simply releasing the game on Series S.
You yourself said that Series X should sell well once first party output starts. Xbox One never sold well and still managed to sell 50 million consoles. In general I don't get how it should manage to sell less than One except if the console space starts to shrink in the near future in general. And nobody is predicting that right now.