"It's complicated. But let's look at things from a more general perspective. There are multiple factors that contribute to this number, which is ever-changing; the most relevant being the 85 player scholarship limit, the 25 scholarship per academic year restriction, transfers, players who fail to qualify after signing their letter NLI (National Letter of Intent), early enrollees and grayshirts.
FBS (Division 1A football) allows up to 85 scholarships, or total counters, per program at any given time, making it a constant factor in the number of prospects a school can add to it's roster, annually. At the same time, schools are only allotted up to 25 new scholarships per academic year, called initial counters.
Players who transfer from institutions positively impact the overall (85) player scholarship limitation by opening up a spot. While players who transfer in take up a spot within the overall 85, but only count as initial counters if they transfer in at the beginning of the academic year (as opposed to mid-year). However, an even more direct component of the number of players a school can attain any given year are early enrollees; which, when combined with grayshirting, yield an over-signing work-around."