The stats on this have been done before but since we are talking something naturally distributed here, starting figures will have the average of say a million.
Then you go up a standard deviation of say another inch and it gets far less very quickly so the number of people then would be 100k. Another inch and its even more extreme, it’ll be 8k.
So by the time you start talking about people 4 notches away from the average it is a couple out of that million at best
That kind of abrupt drop off in variation applies to height where say the standard deviation might be 6 inches, anything nature based pretty much.
That is assuming penis size follows a normal distribution. Very few things follow a normal distribution (I have a PhD in finance).
I would think that there are more 9 inch penises out there because penis size does not follow a normal distribution.