Why set an arbitrary number of launches?
We should launch as many times as we have legitamate missions.
These may include DOD, and civilian research (if we continue on the path of prohibiting ISS from doing much science) missions. If we go forward with ISS science, then STS can be demoted to manned transport duties.
On the other hand, there is an issue with staffing. You need to keep a minimum number of folks employed (whether you launch or not) to be able to support any launches. These people must practice their craft often enough to keep up their competency and to train up replacements.