That is the question, but I would replace "a hit" with "an effective stop".
That would encompass both penetration and the largely random chances of hitting vital parts concealed within the assailant's body or assailants' bodies.
Who can know the answer?
Thats just it, you never know the answer, until its over.
If youre very lucky, one round will suffice, although I seriously doubt, you would really ever know, as rounds two, three, four, etc, would already be on the way, before the effects of one, were ever known.
The only real answer is, you shoot until they are down or you are empty. What else is there? All you can hope for is, you still have rounds left, if they fail to go down on what you had planned or allotted (a major no no the way I see it), and/or, there are other targets you have yet to deal with.
Second, there are some illustrations drawn by a South african radiologist what show the difficulty (or more accurately, the rather low likelihood) of striking anything vital within an assailant's body with any one, or perhaps two or more shots. The link to the source does not work. The anatomical schematics are grossly over simplified, but they illustrate the concept.
I think this brings up something that many probably have not even considered, as many dont really practice "realistically", and base their skills, on what they do on a static range, very often, at either bulls eye targets, or silhouettes, with scoring rings, that have nothing to do with reality.
Reality is, youre going to be moving (well, you should be anyway), and your opponent(s) are also going to be moving, all while youre trying to hit fleeting targets from varying angles, as you go.
Practicing on non descript targets facing "head on", that dont require you to think as to where the "track" to the point that youre "really" trying to hit is, do you no good. You need to know where the important things are, and how youre going to hit them, from any angle you encounter, as youre shooting.
One of the few things of value to come from that idiot box, is practicing just this. If you take the time to learn anatomy, and understand what it is you are trying/need to hit, your TV offers good practice at this, with targets more mimicking reality.
Again, with any of this, the only correct response, is to shoot until the threat is down. As much as the plan in your head says otherwise, it just is what it is, and you have no choice, but to continue to respond, until the desired result is obtained.
I'll see your arthritis and raise you tendonitis too.
Repeated slide racking during a two day training session can certainly leave me hurting for a while.
But problems with a the DA revolver trigger pull make it very difficult for me to achieve combat accuracy while firing quickly enough.
Not only that, but a practice session with a lightweight .38 is much more demanding in terms of recoil than my XD-S 9MM 4.0.
The DA trigger pull on many revolvers is close to what the RSA weight is, on a lot of autos. If you cant work the slide, how well are you going to do "repetitively", with a DA trigger of similar weight? You only have to work the slide once, and thats not going to be under stress, in most cases.
I also agree that the lightweight .38 snubbies, as well as their all steel .357mag versions, are very demanding to practice with, especially if youre practicing with loads similar to what you carry in them. Brutal is a very apt description. Since we really do need to continually practice, how much practice are you "really" getting in?