I clicked on some random post from 2020, 19 em-dashes.
While this writer obviously had a lot of input into the model, they even state (or more accurately according to zerogpt, ChatGPT wrote this whole paragraph) "The writing process should be highly iterative", so they have added their own flavour into the writing, but it is still, (probably not for much longer) but still obvious when this is used.
If you really want to fly below the radar you can even include instructions to adopt a certain writing style, e.g.: you can tell it to use a Gen-Z style with minimal formatting.