org-export-headline-levels

org-export-headline-levels is a variable defined in `ox.el'.
Its value is

3

Documentation:
The last level which is still exported as a headline.

Inferior levels will usually produce itemize or enumerate lists
when exported, but back-end behavior may differ.

This option can also be set with the OPTIONS keyword,
e.g. "H:2".

You can customize this variable.