The Legend of Zelda: Breath of the Wild is currently the only reason to own a Nintendo Switch- the device itself is a shell of its full potential for now, and there’s no other game worth owning on it. Zelda, though, is possibly the greatest game ever created- and many will buy a Switch just to play it.
In Japan, that has included over half the people who bought the Switch. According to Famitsu (via NeoGAF), The Legend of Zelda: Breath of the Wild has sold 193,000 units in Japan on Nintendo Switch over the first three days- this is in line with Skyward Sword, and more than Twilight Princess sold back in 2006. While this is lower than the sales of the DS or Nintendo 64 Zelda games, do remember that this does not count sales of the Wii U version, and it does not count any digital sales.
So, Zelda is, as expected, off to a good start- how long it can sustain its own sales, and also the sales of the Nintendo Switch, is now what remains to be seen.