Ehh, sales have been sort of dropping lately:
Pokémon X & Y: 16.42 million
Pokémon Omega Ruby & Alpha Sapphire: 14.23 million
Pokémon Sun & Moon: 16.17 million
Pokémon Ultra Sun & Ultra Moon: 8.57 million
Pokémon Let's Go, Pikachu! & Let's Go, Eevee: 11.28 million
But I'm guessing Sword & Shield will probably have close to 15 million sales since it will be the first of a new Generation. If 4th Gen Remakes come, considering how popular they were, I bet they could reach close to 20 million.
I don't want Custom Pokemon. I want the creators to work harder. ;_;