At a minimum about 40 per round Which comes to 500 over a distance which is the minimum you look for. Usyk dropped from around 800-1000 at CW which is insane down to 600 or less at HW given the size increase, quality over quantity.
Probably double whatever the current heavyweight average in fact is. (guessing about half welterweight's average) Some heavyweights will whine that their conditioning is too bad to handle that output while carrying 200+ lbs ... well then, up your roadwork and git gud.
No reason they can't throw 40-50 per round. At least after rounds 2 or 3 one they get the feeling out rounds out of the way.
Conditioning is not the only limitation. The fighters at HW punch hard. The fighters in the low weights do not punch nearly as hard. Thus, the margin for error is significantly smaller at HW. If you go in guns blazing recklessly like smaller guys do you are liable to end up getting stopped.
Frazier found out the hard way what its like to be a pressure fighter at HW. It can work against lighter hitters. But its a tricky strategy to use against the big hitters. There aren't any George Foreman's around in the lower weights.
Assuming Ike Vs Tua level sets the gold standard...81 a round for Ike. 63 a round for Tua. Somewhere starting with at least 50 punches a round would be a great average. Takes a chin to pull it off as heavyweights are heavyweights after all.