Your question is misguided. Anyone can enroll at UM whenever they want. Nobody has to "wait to join the team in Fall". If we wanted to do so, we could enroll 25 high school signees in January.
As for the impact of the Initial Counter rules, you can take 25 per year, and Initial Counters include both high school signees and portal transfers and JuCos. We did NOT have "20 entries" in 2019, because all of our portal transfers last year count against the numbers too.
We can take 25 guys this year (whether they enroll in January, June, or August), between the high school signees, portal transfers, and JuCos. That number MIGHT exceed 25, to the extent that we are allowed to add to the number due to the mid-year graduate exception.
So we might hit 26 or 27, all in (HS, portal, JuCo). They can enroll between January and August. Whenever they are ready.