I'm not sure there's a definitive answer.
You've come to the most likely place I suppose -- one place where numerous males are working on that research -- but it's pretty informal. I suppose someone would have to be logging their doses along with doing regular fertility testing... and then the experiment would have to be repeated enough times to obtain statistically significant data.
I would hazard a guess that if you are in generally good health, and eat a good diet, and are a good red-blooded and virile example of a 20-something guy, you'd have to take more than your body could handle.
The forum's research seems to lean toward the notion that you'd likely notice other more immediately unpleasant symptoms well before sterility could occur.
Would you be interested in a golden opportunity to be personally involved in advancing science?
However... let me say, not so fast! You needn't concern yourself with a maximum dose. There seems to be some indication that too much PM can stall progress. Resist the urge to dive in headlong.
The wiser course seems to be start slow, work up slow, and stick with what gives results. The other is tantamount to pushing until you feel poisoned, and that's not usually a good thing. Listen to your body! Your 40-year-old future self will thank you!