Currently at 17.3M shipped, just behind Diamond & Pearl (17.67M), Gold & Silver (23.1M) and Red, Blue & Green (31.38M).
RBG's record is impossible to reach, but considering that SwSh will have DLC to further carry it and since no new games are releasing this year, it should continue to have good legs to comfortably put it above 20M. It has a chance, thanks to its great word of mouth, to become the second best selling Pokémon game of all time.