As others have said, there's good reason to expect 2 characters at E3. Not only has E3 provided at least 2 characters every year for a reveal, but based on the timing...it just makes sense.
So we have that tidbit that was actually sent to players at the launch of the game (Push tweeted about it the last few days) in which it was stated that Fighter #2 and Fighter #3 were being worked on at that moment.
Regardless of who Brave truly is (Spoiler: It's probably Erdrick) they were in the coding at the same time as Joker. This would keep things somewhat consistent. In addition, as of June Nintendo will have about 8 months to release the rest of the characters...not to reveal, to release. They will not reveal the last character in 2/2020 unless it's a similar thing to Joker, and who's to say that can't happen? Regardless, they need to strategize their releases. It only makes sense.
Sure, there's a bunch for Nintendo to talk about. Dragon Quest XI, Animal Crossing, Mario Maker 2, Astral Chain, Fire Emblem, Pokemon, and a ton of others. But I wouldn't be surprised in the slightest if they have a separate broadcast for Smash, just like they did when they revealed Roy and Ryu, and then released those two with Lucas at the conclusion of the broadcast.
Could we get 1 character? Maybe. I think it's far more likely we get two and they're either released immediately following the broadcast, or one is released that day and the other in August. It's doubtful we'd get another extended period of time like we did between the reveal of Joker and the release of Joker at this rate. Something has to give.