It'll probably already have shipped 15 mil by the end of the quarter.

If they don't overshadow it with a new mainline game this year, I could see it crossing 25 mil lifetime.

I feel like there's no way It'll sell less than Sword and Shield as it'll most likely be the first Pokémon game I buy and I see other people that are like me. The sales depend on how long it'll be the main Pokémon game, but I'm going with just a hair over 30mil.

Considering that Sword and Shield (a very mediocre Pokemon IMO) reached almost 24M, I'm calling 30M+. In fact, I wouldn't be surprised if it outsold Red/Green/Blue. This could be Pokemon's MH World moment. At least I hope it is.

I doubt it'll sell over 40 million and I would count yellow with Red/Green/Blue.

As they say above, my vote goes to the +30. It's a very high number, but seeing the success of Sword/Shield and Diamond/Perla Remakes, I doubt Arceus will be below it. Besides, Pokemon has always had a problem: by the time the next generation comes out, the game is already dead in sales. Arceus has completely different mechanics, so this won't happen to it.

Do you also count Crystal, Emerald, Platinum etc. with the previous duo?

With the announcement of Pokemon Scarlet and Purple I lowered my bet to 12-14M... Very bad management of TPC's release dates...

I came here to comment something similar. Baffling decision. They need to slow the releases down. 

Not only the releases, but also the announcement doesnt make sense, timing wise, just 1 month after Arceus release. Will severely cut into its legs.

Agreed, I mentioned in the annoucement thread they should have waited until June during the Ninty E3 equivilent show. Spread the hype out. It's just such an odd choice and timing.