I'm gonna tap into my inner tbone and go 4-5 million. There hasn't been a Yoshi game that has broken 4, or even 3, million since SMW 2: Yoshi's Island on the SNES. And even then, I'm pretty sure a big reason why it sold that much was because it had the name "Super Mario World 2" in front of it.
But so far, the Nintendo Switch has been the system that has provided a number of #1 Best Selling Games in a number of franchises: Zelda - Breath of the Wild, 3D Mario - Odyssey, Splatoon - Splatoon 2, Xenoblade - XC2, Mario Tennis - Aces, and Smash Bros. - Ultimate, to name a few. Hell, it can even be applied to 3rd parties, FIFA 18 is the best selling FIFA game on a Nintendo system, with just the first entry on an install base that is far less than what the Wii had the last time that happened. And if FIFA 19 keeps tracking like it has, then that will become the best selling FIFA on a Nintendo system.
Plus, Kirby Star Allies and Super Mario Party have a shot at becoming the best selling games of their franchises. I expect the same to hold true for Yoshi.