You write as many chapters as you need in order to get your message across. this applies to all types of writing, papers, reports or theses. I am sure some people might avoid 13 chapters or whatever number they feel unsettled about, but that will not have anything to do with scientific writing. Likewise there is nothing that says any form of scientific writing needs a certain number of chapters. That said, the Introduction-Methods-Results-and-Discussion (IMRaD) format, which forms the basis for most scientific papers, leads to four main chapters and in addition an abstract and a Conclusions chapter. Again, this is because it is a standard logical form not a "magic" number.
There are, however, many other typographical rules that influence the formatting of pages and chapters. For example, a chapter should start on an odd page number to follow traditional rules. There are thus many aspects of typesetting that influences the format of printed text but not the number of chapters.