When I was working on voxel terrain generation in Godot, I found that performance was better with one larger collision shape than it was with many smaller ones. In my case, I found a sweet spot between large collision shapes and performant voxel processing in chunks, so I could modify the world but keep the collision shapes fairly large.
Though I would highly recommend you test to know what would work best for your project if possible. In my case most of my collision shapes were close together, which may have a difference and may not be applicable in your case depending on your project.
