It really depends on who you send your email to. After a few reponses, you will know if it is too much or not. A quick response with detailed explaination means your pace is ok. In my experiences, if it takes s/he half a day to response to my email, I would wait half a day after receiving the response to send my new questions.
I ussually put a question or two in my message, whatever unclear to me at the time. i.e.
- How do applicant get on and off the waitlist?
- When is the next round of acceptance if I don't get in this round?
- I finish all prereg but you still mark me incomplete, would you verify my requirement checklist?
- I am planning to take either pathology or histology next semester, which course would you recommend to increase my chance of acceptance at your school? ...
In each messge, I always try to sneak in a big portion on one or more of the following topics regarding my feeling about the school, the interview, the student I met, the curriculum, the PBL, the staffs, how and why USC was and still remain my fisrt choice. I also use this opportunity to explain why I did not perform well at the interview, or anything unclear about my file.
Keep in mind that the clerk that you are communicating with do has the power to make a spot available to you. So, be polite and professional.