Getting an accurate figure overall, for all athletes across all sports at every level, would be a gargantuan task. So I don't know that you're ever going to get a good answer to that.
I think it's safe to say there's probably some variance from one school/sport to the next.
 
I do know it's rare to hear about anyone being academically ineligible these days. And a lot of them have degrees conferred, so I assume a good chunk are still attending class. Probably a larger percentage than the percentage of athletes getting paid to play.
 
The assumption that athletes are skipping out on class is something that far pre-dates NIL. Are more doing it now than 20, 30 years ago? I have no idea. I also do not care. I'm rooting for Austin Myers to rip missiles into the end zone all day every Saturday whether or not he's sitting in HIST 4360: The Mediterranean World In Late Antiquity on Tuesday.
 
I wonder which has a bigger impact on academic pursuits – NIL or the transfer portal. Got to think there are plenty of examples of kids transferring from school to school, losing credits along the way, and being short of hours when their eligibility/scholarship money runs out and never finishing up. Would be interesting data to see.