reftex-toc-max-level

reftex-toc-max-level is a variable defined in `reftex-vars.el'.
Its value is

100

Documentation:
The maximum level of toc entries which will be included in the TOC.
Section headings with a bigger level will be ignored. In RefTeX, chapters
are level 1, sections are level 2 etc.
This variable can be changed from within the *toc* buffer with the `t' key.

You can customize this variable.