It's not about the number of characters involved. It's about the reach of the effects of that RP. The number of people affected usually will become large eventually regardless of how many people were initially involved: one private interaction between two characters can affect subsequent interactions with many, many other characters given a sufficient period of time for those interactions to occur.
Ripples in a pond.
Let me give an example: EVERY single one of Morwen's interactions with other people over the last year or so has been directly influenced by every other interaction (or lack thereof) with other people that came before it in that timeframe.
If, let's say for argument's sake, Esna or Ava wanted to say that, no, their character had actually been around more or had done more to try and interact with Morwen during that period, I'd tell them no, despite them being my friends and despite it appearing on the surface to be just a one-to-one thing. Because even a seemingly private change like that, of one person going out of their way to do something different, in private, could have - and would have, in this case - drastically changed every subsequent interaction afterward, private and public.