Imagine for a minute I am a an extremely powerful mythic hero and I enter a thorp of 50 people. Average farmers, bakers, cobblers, good wives, etc. I institute a policy that I will train every one of them to be an adventurer and eventually reach a level where they can do an impossible task. (Cleave a mountain, divert a river, swim up a waterfall, etc). I have the power and resources to train them all and keep them alive until they reach said level. Additionally, they are slavishly devoted and will do everything in their power to accomplish this goal.
How many of those 50 people will be able to accomplish that impossible task? One? Ten? All fifty? Zero?
The answer you provide is essentially my concern with this system. If everyone is able to do impossible things with sufficient time and devotion, then the world should be full of people doing impossible things. Not everyone, but lots of them. If we say not everyone has the spark needed to reach that level, then we have to say why. Is it genetics, magical aptitude, supernatural blessing? Why are some able and other not?
Hercules is mythical because he's the only person in ancient Greece who can do that kind of epic stuff. There are other great heros, but none of them can divert a river. Jason is favored and Odysseus clever, but neither are matching Hercules in power. He's got some great parentage to thank for that.
Which returns me to my original question: how many people in that thorp have the potential to be Hercules?
How many people in that thorp have the potential to become sorcerers?
Any character can take a level in sorcerer at any time. Does that mean every person is descended from every possible sorcerous bloodline? If so, why does anyone ever bother doing any other form of magic? If not, how come any character can take three levels of the class and then pick any bloodline?
The fundamental answer to these questions is the same as the fundamental answer to your initial question: It cannot be answered in the generic because there
is no answer in the generic. We have no way of knowing how many of the people in that thorp might cross that line--or indeed
any line--purely through training. Perhaps in some settings or some villages, the answer is all of them. Perhaps in other settings or other villages, the answer is none of them. Perhaps there's some insight or revelation or achievement that does it. Perhaps having to endure unexpectedly brutal trials and tribulations is the only way to find out--which means the
entirely artificial "training" you speak of here cannot ever produce such a thing, only the actual rigors of adventure can. Plenty of things work like this in real life, things where no amount of structured, artificial, isolated processes can replicate the real deal. As an example, there are nickel-iron crystal structures you can find which prove that the metal must have come from a meteorite, because those crystals cannot form on Earth (they require the extreme cold, microgravity, and nil-atmosphere conditions in space)--even though said structure is totally mundane, it is impossible to artificially create on Earth, no matter how hard you try. (For reference, they're called "Widmanstätten patterns", and as noted, they require conditions which
cannot occur anywhere on Earth. Even the extremely rare rocks that
partially resemble these meteoritic crystals will always be clearly distinguishable from the real McCoy.)
Or, for the pithy referential answer, "There is as yet insufficient data for a meaningful answer." The question cannot be answered in the way you have structured it, because it does not have answers of the kind you're asking for. It has other kinds of answers, but not that kind.
Why does lightning strike one tree, and not another? Why does one dead dinosaur fossilize beautifully, and another rots away, never to be seen again? How many roads must a man walk down before you can call him a man?