I guessed 27m - 27.9m. I could easily see it selling both higher or lower though.
Two things that are still unclear to me are what effect Arceus will have, and also how Game Freak is affected by Arceus. Until now Game Freak's release schedule has been quite predictable. Let's say Sword/Shield is analogous to X/Y on the 3DS, since they are both the first new Pokemon on the system. Then Brilliant Diamond/Shining Pearl is analogous to Alpha Sapphire/Omega Ruby.
What is analogous to Sun/Moon? Right now I am assuming it's an unannounced game coming late 2022. That is what my guess is based on. But what if Game Freak thinks Arceus is analogous to Sun/Moon? Or maybe they don't think Arceus is analogous to Sun/Moon, but it's development still delays the release of the next Pokemon game? Either way, I can see Arceus affecting Game Freak's development schedule so that the next Pokemon is not late 2022, and if that happens then my guess is going to be off.