They have some data on this (
https://www.irs.gov/pub/irs-pdf/p55b.pdf, pages 33-35). Out of 509,917 examinations, 18,988 resulted in a refund. There was $7.0 billion of tax refunded in 2020. For comparison, there was $17.2 billion of additional tax generated with additional recommended and unagreed amounts.