org-n-level-faces

org-n-level-faces is a variable defined in `org-faces.el'.
Its value is

8

Documentation:
The number of different faces to be used for headlines.
Org-mode defines 8 different headline faces, so this can be at most 8.
If it is less than 8, the level-1 face gets re-used for level N+1 etc.

You can customize this variable.