Not sure, but would GUESS that they are interviewing 16 (the 10 offers plus 6 waitlist). 30 seems nuts, faculty around here are already crazy enough with admissions stuff. Since they have historically made about 23 offers, 30 interviews for a smaller cohort seems illogical.
Clarifying note from a University of Chicago English third year -- last year's yield was too high. 23 (or so) offers were made for an ideal cohort of 8-10, but 16 accepted. Very expensive for the division. This year, to better control the cohort size, they are making ten offers with six waitlisted (also a first). That's why interviews are now part of the universal selection process in English.