Average means nothing in this instance.
Because an average is a compilation of ALL shootings, some of which will be .22’s, .380’s , .32’s, .38 specials, all the way up to 8mm, ,45’s, etc.
There will likely be more 9mm than anything else and that alone will distort the number of shots required.
In addition to caliber, to get the truth, one must know what load was used. Bullet construction is as important as weight, caliber and velocity.
For instance, the proper bullet in a 9mm will be more successful than the incorrect bullet in a .45.
The only way to determine the most effective self defense load is to take each and every shooting, determine the number of hits, WHERE the HITs entered the body and the result...dead, total collapse from broken bones, or only convinced it was time to give up.
Studies on defense against grizzly and black bears have been done that way on a very small scale, simply because compared to shot humans, there are very few.
That comparison makes it very clear that the bigger the ammo, the better the shooter’s chances.
Beginning with the .357 mag, one shot stopping the bear becomes more likely, although to my surprise the .357 barely makes it. The .45 Colt, 8 mm. .40, .44mag. .45 auto and from there on up have many one shot successes on record.
You can argue caliber all you want, no one goes grizzly hunting with a .25 or a .32. Even a .308 is considered an under powered rifle for grizzly.
Little bullets make little holes. Big heavy bullets not only make big holes, they break bones.
No it is the average by law enforcement shootings. .380 is 2.4 9mm 2.7. No law enforcement is using .22.
Three bullets hitting the threat on average to end the threat.
Sometimes it’s 1 bullet sometimes it’s 5 or more.