That's usually the mixed thing you know that trainees are under contract?
so there is no way that one or another agency would have 120 male trainees of the same age or even 120 boys who are 15-16-17 years old. He must be talking about pre-audition inside SM building, which is normal because every agency has few rounds of auditions, the difference is that not all of them hold them inside company building.
Then there are situations like trainee A comes to dance class and he sees trainee B, but trainee B is 7 years younger and just joined, so they aren't in the same "batch" for one group.
The list of JYP trainees was once circling around the web, and there were maybe 45-50 people listed but it was both for boys and girls, and like I said age gaps between them were big so it was clear that teachers/officials ain't targeting same group debut for all of them.
The other way you can think about it it's like classes in a school in a very small village. The thing is you don't know how many babies will be born, so later in one year only 4 kids are in class, but a few years later there is suddenly a class with 8 students. And next year will have 6.
This is how it works, and only the luckiest are given the chance to debut just few months after joining company, sometimes there are also those unlucky ones who join company few months after new group debut, and they are working their asses off just to stay in company for 5-7 years and not debut because their trainee contract ends and there are no plans to debut new group, or suddenly company changes concept to debut 14-15 year old kids.