title_body: Can I expect one chapter per html file in an Epub?
upvoted_answer: No it is a valid file if there are several chapters in one file. You can even have the whole book as one HTML file.
  
  The epub spec says
  
  
    Each Rendition of an EPUB Publication contains one or more EPUB Content Documents
  
  
  Book marks can be made to a point in the file so don't need separate files.
  
  Readers have a limit to the size of the HTML file and I think that is a reason to split the files.
  
  Again with movement in the files it depends on the reader. Having smaller files might well help the reader as it can cache a whole file