California resident or not, MSTP applicant usually applies to ~15. Some schools like UCs tend to admit California residents, but I believe such rules don't apply for MSTP admission. Thus the average number of MSTP a CA resident applies to is same as that of people from other states.
Since MSTP applications may include more essays than MD applications, doing more than that will exhaust you. Doing those secondaries seemed almost like a part time job. Good luck.