I only know that changing the number of sections (to display) does not actually delete their contents. They are only hidden or shown depending on this number.
You can have a look at the xml-file in the course-backup-file to see whether the content of the course sections are backed up in any case. I guess that contents of the sections has to be present in the course-backup as well, as after restore of this course you would expect the contents of the hidden sections to be shown if you change the number of sections to show. Losing content through a backup does not make sense as long as the content is actually present in the original course whether displayed or not. But backup-page might show them as hidden/empty.
Rosario