This is kinda hard. So far we got sequel to home console games. Kirby is the first handheld game coming out. We do not know if the Switch will boost handheld games like it does to the home console games. Kirby's Return to Dreamland did 1.66 million on the Wii back in 2011. Triple Deluxe did 1.93 million on the 3DS. Hm
I am going with 2.5 million assuming it is a good game.