org-cycle-max-level

org-cycle-max-level is a variable defined in `org.el'.
Its value is

nil

Documentation:
Maximum level which should still be subject to visibility cycling.
Levels higher than this will, for cycling, be treated as text, not a headline.
When `org-odd-levels-only' is set, a value of N in this variable actually
means 2N-1 stars as the limiting headline.
When nil, cycle all levels.
Note that the limiting level of cycling is also influenced by
`org-inlinetask-min-level'. When `org-cycle-max-level' is not set but
`org-inlinetask-min-level' is, cycling will be limited to levels one less
than its value.

You can customize this variable.