I'm not going to say no. But I would like to know why you think it will sell 20m+
Sure, i think 20mil first most and most obvious because the game will be achievable on such a system unlike 3ds and especially wiiu.
20mil because with the inclusion of inklings, japanese market will grow compared to that of past entries. There for possibility of a 4mil seller is there compared to 3ds and ssbb 2.5mil ish.
More importantly there are alot of new nintendo owners and that of a mature audience and smash is a perfect market for that specifically. With how much bigger mario is with botw link that have gaves that tower over their previous entries the game will look even more attractive imo. Add in to the fact that this will be more popular to have in handheld mode compare to 3ds version and being optimistic i think it can happen