Performance wise some old posts mention | Official Unity Discord | Page 1

I'm afraid that is going to be a feeling decision.
You can make master shaders for things that will share most render features but differ by input (safe to say that unless your game as special needs for some character, you can have all character, probably all skinned models, using the same master).
At some point it becomes a readability issue (massive graphs can get difficult to work in).
An example of something you'd exclude from a master material is water, as it's likely to require a lot of custom features (on the other hand I would isolate the features that are shared with the master into their own include hlsl file or custom nodes (if these work this year in unity))

Unity and Unreal sort of do this already, the inbuilt standard shaders, like LitSurface shaders and the like have a ton of option that you can toggle, they are template masters in many ways.
The master node in shader graph (as it's name indicate) is something similar under the hood, conditionally compiled hlsl code template for common features such as transparency.
It's a good starting point to help decide what should be in a master shader

#Performance wise some old posts mention