I just looked at NCAA reported numbers (www.ncaa.org/about/resources/r...transfers-division-i). From 2007-2018 the transfer rate for D1 baseball (all transfers (2-4, 4-4) went down from 27.6% in 2007 23% in 2018, with 2-4 transfer staying fairly static around 20%, while 4-4 transfer reduced from 8.1% to 2.2%. So this is nowhere near the 50% that RJM mentioned, however I also think the data is really not representative of the entire transfer picture. What the data does not show is the transfer rate leaving D1 going to D2, D3, NAIA and JUCO, this seems to just be inbound from 2 year to 4 year D1 and within 4-year D1.
Anecdotally, and this is just recent memory here (last couple years) of the kids I kind of followed who went D1 or D2, the “studs” seemed to stick at their first school and the other guys (4 of 5) transferred. Two have gone 4-4 within D1 (one waiting on eligibility waiver for this season), one went D1 to JUCO, the other went D2 to JUCO. The studs are all high round draft prospects and major contributors on their P5 and top mid-major programs.