This table below can give you a solid outline.
https://www.aamc.org/download/321508/data/factstable24.pdf
Out of 140,000 applications, if you want to define the cut off for competitive as having a 3.4+ and a 30+ that includes 60,000 people. So right off the bat, about 55-60% of people who apply have stats lower than that. The acceptance rate for those with stats lower than that will be low; think somewhere a bit under 20%. Number goes up to 75k if you include people from the 3.6-4.0 and 27-29 group.
But it's not unreasonable to assume that at an average school, given these stats that at least half of applicants are going to have stats that make it a long shot to apply on average. Keep in mind, even a 30-32 and 3.4-3.59 might not be all that competitive or a 3.7/28 might not be competitive.
There are other factors to consider as well. This table does a fair job highlighting the level of cluelessness of many applicants when you see schools like Robert Woods or FSU get the majority of applicants OOS when only 2% of their class is OOS. That only adds more people who aren't really competitive applying; people with fair stats will apply to schools with really strong IS bias or schools who's mission doesn't fit them.
https://www.aamc.org/download/321442/data/factstable1.pdf
And then on top of that you have to consider the applicants who have clear flaws in their app. Limited clinical exposure. Minimal volunteering is another big one. Some other ones such as blatantly poor secondary essays or lukewarm LORs are other variables to consider. All these variables keep adding up be it poor stats, to applicants with decent stats applying to schools that don't fit them, to having flawed EC's or other factors to the point you will get a large number of applicants at a particular school who probably are well below the average matriculant at that school. How much of the applicant pool is that? I don't know. But at a given school it wouldn't surprise me if something like 2/3 applicants at least really had no business applying there if I were to make a wild guess based off what I said.
If you talk about top schools, about 5,000 applicants a year either have a 3.8+/33+, 3.6+/36+, or 3.4+/39+. Those tend to be the type of stats that are competitive for those schools. And obviously you can be assured very few with those stats apply to all top 20's or maybe even half the top 20's. Remember the average applicant only applies to 13-14 schools. So if you want to make the big assumption these type of people on average apply to 6-7 top 20 schools well a given top 20 school is probably only going to have 1500 applications like that a cycle. IF the average top 20 school gets 6000 apps a year, that means really only about 1/4 has those type of stats that make them truly qualified. Obvoiusly there will be some who can be competitive without those stats such as URMs, but that at least gives you a ball park estimate.
