How many chapters are in a book?

If you want to start a book that isn't divided into chapters, it really is your book. You can design your book however you want.
Note that you may get some pushback from editors: Terry Prachett used to write his Discworld novels in chapterless form, and despite the fact that by the 15th one or so it was pretty well proven that they did in fact sell quite well in that form, he still got pressure from his editors to include them (some of his latest novels do have chapters).