You can check out the Waitlist/Appeals discussion from the last few years. Here is the appeals data from 2023:
2023: APPEALS: 729. APPROVED: 0
https://talk.collegeconfidential.com/t/uc-san-diego-freshman-class-of-2027-waitlist-appeal-thread/
https://talk.collegeconfidential.com/t/ucsd-class-of-2026-waitlist-appeal-discussion/