This is such a good question, and as with everything else, "it massively depends" is the easiest answer. That said, I think we can do some basic maths to at least attempt to answer it - thread below
All publishers are different, but if we take the attached image (from a talk I gave in 2020) as an example list, you can start to see how much work might actually be involved to be a publisher.
These are NOT things a publisher does themselves, but things that need doing.
