Okay, this is pretty interesting. Either Amazon grabbed a number out of thin air, or they know a bit more info than we do. By all accounts, Smash Bros. Ultimate has seen 68 fighters revealed for the roster. That number isn't counting echo fighters, which would bump it up to 74. If the Amazon listing is correct, it seems that number would pertain to standard entrants and not echo fighters. That would have to mean 3 more fighters are yet to be revealed for the game. Certainly interesting, isn't it?!
UPDATE - I just wanted to add that we're seeing all kinds of math for the roster. Echo fighters, Pokemon Trainer counted differently, and all kinds of mixes. There doesn't seem to be a definitive number yet, but according to Eurogamer's count, we're at 68.
UPDATE 2 - Amazon has now removed the number from their listing.