As much as you need.
I’ve fit rendering jobs into 4GB of VRAM and I’ve had rendering jobs die when they needed more than the 16GB I had available.
Not every rendering task is the same and not every modeling task is the same.
Rule of thumb: The more complex your models and the higher resolution your textures,
the more RAM you’ll need. Basically, the more you throw at it, the more you need.
I don’t generally recommend less than 16GB for most people working in 3D and 32GB systems are so affordable
that it’s pretty reasonable to go for it. Beyond that…. if you need it, you need it.
The last thing you want to do is have a system paging… paging will kill your render times.