Things like the Rule of Two were just made up as an explanation for why Vader and Palpatine were the only ones in the OT.
The Rule of Two could be recontextualized to be just the nature of the dark side, instead of an arcane rule the Sith must follow. Like, masters just tend to pass on their knowledge to only one apprentice, and the lack of multiple apprentices means that over time, the number of dark side masters dwindle to one.