Is this the rule or can I find several chapters in an html file? Having 1 chapter helps moving back and forth chapters or set bookmarks.
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
