We don't even have a concrete definition for what conscious means but we use that classification as a means to make rules about what we can and can't do to certain entities. Can we destroy an ecosystem that has been found to be full of conscious mycelium? Is that worse than if we determined it was not conscious after all? Without a concrete definition of what consciousness is, we assign consciousness to things based on feeling and on what we want to signal to other humans about these entities.
Understanding what is conscious is and how it comes about seems interesting. I think currently it falls more into the philosophical sphere of science rather then biological or physics. Maybe once more is understood about human consciousness we will be able to categorize and add other life forms and consciousness types to this list.
It's all very fascinating but it seems to break down very quickly as the definitions mean nothing.
I think it's pretty likely a collection 10x the size would have much more than 10x the complexity due to the interactions. In the same way as a quantum computer, the state of each collection of cells is not quantized because it has many analog values at different ratios and interacts in complex ways with the surroundings.
Of course by this logic a collection of phytoplankton is complex because I'm sure they all have chemicals they excrete. But I also kind of honestly think a rock is conscious by any reasonable definition, so I know I'm not the one who should be drawing the line...
Mushrooms are so different than anything we know it's hard to rule out what they are or are not doing. There are mycelium networks that span massive spaces, 10s of square kilometers. There is a lot of mass, a tightly interconnected network that senses, computes, and changes its environment. So its hard to really rule anything out yet.
Honestly, I think there is a lot of complexity happening on this planet that we are missing, especially if we open ourselves up to larger timescales.