There is no usually, really, since it's just a big mash of different authors with different styles writing different types of stories. Generally speaking, it should be at least a few endings, but it really just depends. If there are a lot of endings, I would prefer that they have distinct paths leading to unique endings and not just a few small differences. It would be boring to have 21 endings, but each one just has a different flavor of ice cream.
I agree, this is pretty hard. Writing a CYOA is so different from a linear story, but it's also really awesome to explore different paths as a writer. I also really enjoy having an obscure story that leaves people wondering what the true ending is.