Warning! The following contains major spoilers for Star Wars: The Last Jedi!
In an article on Snoke in the new issue of Star Wars Insider magazine (which is an official Lucasfilm publication, so consider this canon), it's mentioned that Snoke has trained "at least one other apprentice" besides the former Ben Solo. Slashfilm caught this little nugget of potentially explosive info, and helpfully scanned the page itself.
Of course, when we last saw Snoke, he was not looking his best, having been betrayed and sliced in half by Kylo after the latter learned that his mentor had been manipulating Kylo's conversations with Rey (Daisy Ridley) over the Force version of Skype (aka Force Timing). That means that any other information we might have learned from Snoke himself has died along with him -- much to some fans' chagrin.
If the article is correct and there is another apprentice out there, that could set the stage for this yet-to-be-revealed student to step forward and want to claim the Supreme Leader title from Kylo, who promoted himself to the job after killing Snoke. Or the unknown apprentice might simply be looking for revenge, which would also make Kylo's life a lot more difficult than it already is.
Where would this apprentice come from? Would he or she be a relative of Snoke's? A Knight of Ren? That's presumably up to J.J. Abrams to figure out if he pursues this story thread in Episode IX. Clearly Snoke can have as many apprentices as he likes, since he's not even a Sith with their "one apprentice per customer" rule.
Or... this could be old information that was set for publication before perhaps even the writer knew what Rian Johnson's plans were for Snoke in The Last Jedi. It might end up being an intriguing yet ultimately unexplored narrative path -- we know Abrams likes to fool around with those once in a while. What do you think? Is there another apprentice lurking out there who might get up in Kylo's grill in Episode IX?