Looking at ESPN top 25 players over previous years, here are numbers for the 2017, 2018, 2019, and 2020 classes:
2017: 5 of the top 10 players transferred, 2 went pro early, so only 3 of the top 10 suited up their senior year for the original school. 13 of the top 20 overall weren't at the same school as seniors (11 transfers, 2 early pros)
2018: 5 of the top 10 players transferred, a 6th went pro early, 11 of the top 20 overall weren't at the same school as seniors (10 transfers, 1 early pro)
2019: 5 of the top 10 players have transferred, 11 of the top 20 overall have transferred
2020: 5 of the top 10 players have transferred, 9 of the top 20 overall have transferred (and 13 of the top 25)
2021: none so far (granted the season ended 4 days ago)