My guesstimation is 18 to 20 million. I doubt it will beat Gold/Silver since as others have mentioned, there's a new Pokemon game every year and also because at present, it doesn't look ambitious in it's scope.
Since Let's Go already hit 10 million, I would say it's safe to assume Sword/Shield will nearly double that amount but maybe the nostalgia value is far more stronger for Let's Go?