It's been made clear that Supreme Leader Snoke is not a Sith and that Kylo Ren, while powerful in the Dark Side of the Force, doesn't seem to have much interest in being one either. In Star Wars: The Last Jedi, the former Ben Solo murders his "master" and that means there's no longer two as Darth Sidious once said should always be the case. However, it sounds like there could be another apprentice!
According to a companion magazine for the Rian Johnson helmed release, "Force sensitive, and highly attuned to the dark side but not a sith, Snoke has trained Kylo Ren and at least one other apprentice." The question is, did he train that mystery apprentice before Kylo Ren was turned or is there another warrior out there with ties to the Dark Side? There are a real myriad of possibilities.
A future Star Wars movie filling in the gaps between Return of the Jedi and The Force Awakens could introduce us to a new apprentice Luke and company clashed with or Episode IX may very well bring in someone looking to make Kylo pay for killing Snoke. Either way, this throwaway reveal could have some huge ramifications for the Star Wars franchise moving forward so we're going to have to wait and see.