What percentage of admitted PhD students comes from waiting lists I was wondering if there is a rule of thumb for the following.

What percentage of students that are accepted for a PhD position were initially put on a waiting list, and were only admitted after high-scoring applicants did not enroll?
This will depend upon the department, but I would expect that relatively few students would be admitted via a waitlist.  Departments usually have a pretty good estimate of how many students who they accept will take their offer and enroll in their program.