Now that Star Wars: The Last Jedi has finally premiered in theaters, it is safe to say that the Star Wars franchise is currently in a considerably different place than it was at the end of Star Wars: The Force Awakens. The film raises a ton of questions that we didn't expect to have, and it tells us that many of our theories about Snoke's identity did not matter nearly as much as we had assumed -- mainly because he is now in two separate pieces on the floor. We're left wondering quite a bit about his origins, but now it seems that we have confirmation of another apprentice before his time with Kylo Ren.
In a newly released issue of the Last Jedi Visual Companion Magazine (via Reddit), it has been observed that a portion of Supreme Leader Snoke's origin not mentioned in the movie has finally come to light. Specifically, it has been established that "Snoke has trained Kylo Ren and at least one other apprentice," thus implying another product of his teachings somewhere out there in the galaxy. The revelation doesn't go much further than that, but it does let us know that his influence spread to more than just Ben Solo.
Building off of this development, we will have to wait and see if it is expanded on in Star Wars: Episode IX. As mentioned earlier, Supreme Leader Snoke left the world of Star Wars as abruptly as he ended, which means much of his backstory remains on the table and ready for exploration. With one-third of the current Star Wars trilogy left for J.J. Abrams to come back to, we could very well learn about Snoke's other apprentice in 2019, and learn that it's someone else out there in the galaxy. The possibilities are seemingly endless, ranging from Rey (perhaps she just can't remember?) to Emperor Palpatine to Dark Plagueis.
Then again, there's always the possibility that we may never actually learn anything about Supreme Leader Snoke's other apprentice, and that this revelation is just there as a piece of backstory to serve the greater Star Wars mythology. After all, there are plenty of aspects of the Star Wars lore that don't get mentioned in the films (if you know anything about the Expanded Universe, you likely already know that), so this could just serve as a fun factoid to help flesh out the world.