Yeah, what 22and25 said. There's no one specific number that you reach that is all of a sudden bad. One individual's capacity to throw, say, X amount of pitches, does not mean individual Y and individual Z will be able to throw X amount as well. And these numbers can change based on a variety of factors, it's a dynamic system approach.
https://www.mlb.com/pitch-smar...uidelines/ages-19-22 might have a general ballpark to what you're looking for, maybe.