There are only allowed to be 2 Sith.
Star Wars Rebels got around this be creating 'Inquisitors', dark force users working for Vader and the Emperor.
Then there was Darth Maul who even though chopped in half and fell in a shaft at the end of Episode 1, came back in the Clone Wars and Rebels...but not as a Darth.
And now that Disney is making money by the mintload through these movies, you can be sure that there will be movies starring surviving Jedi.
You can’t have more than 2 Sith in a group, because they inevitably succumb to infighting. But they’re out there somewhere. Maul is really the example of real Sith, he’s out there, looking for stuff and power. I suppose when you have a whole Empire to run it takes away from quest time.
Oh sure, there’ll be surviving Jedi, and Sith. But they’ll be even. Which was my point. The last thing the Jedi should have wanted was balance, they ruled the freaking galaxy, there were thousands of them, there were some rumors of Sith but nothing concrete and certainly not enough of them to be a threat. Any smart group of people in their position that thought this kid was going to bring balance would have killed him a lot.