The way Switch software is going, I would not be at all surprised if Smash Bros becomes the highest selling entry in the series.
That shouldn’t be hard at all. In terms of shipments:
SSB (N64) - 5.55M
Melee (GC) - 7.41M
Brawl (Wii) - 13.27M
4 (3DS) - 9.24M
4 (WiiU) - 5.34M
If it releases in September like some people guessed, it’ll easily outsell the N64 and GC entries before the end of 2018. By the end of the next fiscal year (March 2020), it could outdo Brawl and even 4’s combined total (14.58M) with the way the Switch is moving software. If the game is well-received, the sky is the limit.