What happens if I’m on a waitlist but adding the course would go over the allowed maximum per semester of 19.5? I don’t want to drop a course and need this waitlisted course.

At this point, you need to make a personal decision. The system will consistently skip you over if adding the course will put you over the 19.5 credits. You can decide to drop another course, drop off the waitlist or petition for Excess Credit permission.