This is where the attrition rates on CBI's datasets can be useful, especially compared with transfers into the program. Some schools have 8 freshmen leave one year, but 0 the next, so clearly it can vary. But some are more consistent in one direction or the other (as was noted for the Ivy League set).
Also, those datasets are, I think, based on rostered players. So, "preferred walkons" who might commit on PG but never make a spring roster, may not be being counted.