We know that Kylo Ren is Supreme Leader Snoke's apprentice, but recently I was wondering: Did Snoke have any other apprentices before Kylo Ren?
Snoke has had at least one other, and potentially several other apprentices over the years.
The Star Wars Insider Last Jedi: Special Collector's edition contains the following cryptic note
"Force sensitive, and highly attuned to the dark tide but not a Sith, Snoke has trained Kylo Ren and least one other apprentice."
And the Visual Dictionary for The Force Awakens states that Kylo Ren is the most gifted of (presumably) several apprentices.
"Kylo Ren is the most gifted apprentice of Supreme Leader Snoke, a mysterious figure steeped in dark side knowledge who commands the First Order from a distance."
The Rise of Skywalker has now shed new light on Snokes backstory.
Snoke is not a real being but is in fact a facet of the emperor Palpatine, it is not yet understood exactly how Palpatine controlled the being that is Snoke but it is strongly suggested that Snoke is in fact a clone and acted as either the puppet for, or was controlled by Palpatine. Therefore strictly speaking yes there where other apprentices, the most famous being Darth Vader.