I realize there's going to be a bit of variation here from one author to the next and depending on length of the novel, but the range shouldn't really be too large.
The thing is, I really have no idea where the middle of that range lies. 5,000 words? 10,000? 20,000? I know the average adult vocabulary (in English) ranges between 20,000 and 30,000 words, although I don't know if that number includes variations (like, likes, liked, liking), but I would expect this number to be significantly less than the full breadth of a person's vocabulary.
I had trouble finding any information. Is there anyone who is familiar with this subject?