The Switch would need to sell well over 20 million lifetime in Japan like the 3DS which I see very achievable since it will be the only dedicated gaming handheld on the market there in the next year or so, but even then 5 million is outselling the best 3DS game in Japan Animal Crossing New Leaf which sold 4.5 million going by VGChartz numbers, so... right now as bold as I can go is it selling over 3 million in Japan but who knows, anything can happen.