Justin thought he could teach 2 at a time; and we don't really know the upper limit.
As I said, Harry and Elaine could probably each manage 2. Three at once, maybe, but that would be pushing it. It takes
years to properly train a Council-level Talent, and do it right, and it takes a lot of attention. This would be especially true in the case of Talents who had either already started to go wrong, or who had the native personality to be difficult.
The more apprentices you try to train at once, the less attention each one can get, the less personalized the training can be, and the more chance of something important slipping through the cracks.
The Craftmaster/Apprentice model is usually thought of as 1:1 or at most 1:few, but the "vocational school" model shows it doesn't have to be that way, and the WC's "baby warden school" in the desert showed that more-modern "team teaching" methods can get better than a 2:1 student:teacher ratio (though I don't think we have a precise number).
The vocational school model doesn't work for Wizard training. There isn't just a set specific body of skills that have to be taught, at that level each student has their own requirements and has to be taught individually. It's also about philosophies, attitudes, the moral aspects of magic, proper habits of thought and necessary self-discipline. It's about the master recognizing the particular weaknesses and strengths of the student, it can't be entire standardized. What worked for teaching Molly would fail teaching someone else, and
vice versa. What worked well for Harry would not have worked well for Elaine, and vice versa, because of their different personalities and native strengths and weaknesses.
I'm sure you could teach some of the basics in a group-class setting, but as soon as you started getting into the high-end stuff that model would fail.
The baby warden school was just that, all they were doing was trying to accelerate specific
Warden training, not the overall training to make a Council wizard. And even there, it wasn't doing as good a job as they would have liked, it was just necessitated by the war emergency.
"A few hundred a generation" (call it 400 for an estimate (mainly because the numbers work very-cleanly) is only an annual class-size of about 20 per year; but that's worldwide (and as you note, they will miss some).
But even if you assume just 20 a year, you still need at least 7 full Wizards to teach them, assuming 3 per master, 10 Wizards if it's 2 per master. Some of those students will be hard cases who absolutely need the full 100% attention of a master, which makes it worse. Harry does not have even 7 full Council level Wizards to do the teaching. In practice, to teach 20 students would probably realistically need
at least 10 Wizards to master them.
And that's just the first year. A year later you get another 20, but the Wizards from last year are still teaching the first round of students, Harry now needs 10
more Wizards. Let's be conservative/hopeful and say three years can turn a new student into a Wizard. (I suspect it usually takes longer, but let's be optimistic.) That means you need
thirty full Wizards, with the right mindset and skills for teaching, to get 20 students a year through the process. In the fourth year the first round of students 'graduate' and the first ten masters can take 2 new students each.
So Harry needs, at a realistic minimum, 30 skilled Wizards to teach 20 students a year for 3 years a student. That's not ideal, that's minimum, ideally he would want twice that many to really do it right.
Plus he still needs the equivalent of a force of Wardens to be the enforcers, too. That's separate of the teaching staff.
Molly is also specialized, though less-so than Mort.
Molly has specific talents, yes. So does Harry and any practitioner. She's still a full Council-level talent and has vast potentials Mort will never equal, outside his one narrow specialty.
Mort, however, is a world-caliber power... despite his narrow specialty, he's not a "minor" talent.
No, he's a major-level sub-Council talent. His abilities by themselves are not sufficient to make him a world-level player.
Mort took down Capiorcorpus.
Harry tried.
Molly tried.
Mort did it.
No doubt. It doesn't matter. That fact that he's better than they are in that one narrow area doesn't make up for their vast superiority at the other 95%. I'm sure Binder is better than Harry at his one specialty, too. Victor Sells could probably teach Harry a few things about sex magic. That doesn't make Victor a peer of Harry.
Etc.
It wouldn't be perfect -- far from it!
But Harry & allies & the Paranet -- making a concerted effort -- would already be doing more good the entire White Council.
Um...no. The Council is still doing vastly more good at a large scale than they could do.
We see the Council's negative side because we see it through Harry's eyes. His first encounter with them was them putting him on trial for his life for defending himself. He hates the very idea of executing children for breaking rules they didn't even know about. He hates the Council's elitist tendency, even as he recognizes the necessity of it. He really really hates the Council's hypocrisy.
BUT...over the years Harry has reluctantly been forced to admit that a lot of the stuff he hates is necessary. Also, the Council and the Church are the two main factors that have enabled civilization to rise as high as it has in the last few centuries. The Council is the main reason why the average mundane doesn't believe in supernatural monsters anymore: the Council has imprisoned/destroyed most of the worst of them, and forces the rest to keep their heads down most of the time. The Council is the main force that kept the Red Court, the White Court, and some degree the Black Court from running unchecked. The Enlightenment was a Council project that got somewhat out of hand.
Plus, of course, Kemmler.
Even Karrin had to admit, in a backhanded, resentful compliment, that the Council were running themselves ragged keeping the world from blowing up in the instability that followed the fall of the Red Court.
As frustrating and hidebound and hypocritical as the White Council is, the world of Dresden would be a far, far worse place without them.