How many chapter in 1 volume?
Uhhhhhh
It depends on what you are aiming for.
I would say that 40k words is a short novel while 80k-120k is around the amount of words in an arc of an Eastern Fantasy as an example. However, I have seen 20 chapter volumes while I have seen 120 chapter volumes so it really depends on the author, how many words per chapter, etc.
As a general of contracted books, I see around 60 chapters per volume on WN though so if you are planning to remain with the general signed books and what they do, I would aim for minimum 60 but it is not like there is a golden rule for volumes.
Hope this helped,
-OmniFrog