I was talking to an editor using their customer support (it's in Inkstone) and they said that the number of books you can have has a limit.
The amount published can only be 20 (the books have to be in 'serialization')
The amount of unpublished draft books is about 20 as well.
I'm not sure how many 'paused' books you can have but I'm assuming 20?
If anyone knows, feel free to correct me.