A similar request came up recently, where I suggested guestimating these values, since to my knowledge these values are not easily accessible. See answer below:
Since you’re not dealing with the entire vault you might put together a script to do the actual count, but I do believe for most purposes the guesstimate would serve you well enough with changing the variable according to your setup for the size of the frontmatter and a proper ratio related to markup versus actual text.
These values can be found/calculated by comparing the output of the query and the actual word count of some of your chapter notes.