I'd be shocked if the punch total was higher than - or even close to - the Clottey fight. 1200+ punches is ridiculous, even for Pacquiao, but it was only possible because Clottey never fought back. He was a punching bag, hiding behind his guard all night. Mayweather will clearly have to throw at some point in order to win the fight, whereas Clottey's only concern was to live long enough to see the biggest payday of his career.
The important thing about that fight is it provided more empirical evidence on Pacquiao's stamina; it's unparalleled. He still had to throw those punches, regardless of whether or not the fight was competitive. Clearly, he has the potential to throw at high volume and not gas himself, so that won't be a limiting factor when trying to give Floyd his first loss.
In his four fights at or over 140 lbs. (DLH, Hatton, Cotto and Clottey), Pacquiao has thrown 2,723 punches in 34 total rounds; he landed 879 of those punches. That's good for a 32% connection rate, which is just about average for the welterweight division. However, it should be noted that the Clottey fight really skews these stats. Clottey rarely came out of his guard and Pacquiao was only able to land 246 shots (20%).
So let's look at his stats before the Clottey fight; 22 rounds, 1492 punches thrown, 633 landed, for a connection rate just under 43%. That's an average of 68 punches per round, 29 landed per round. According to
CompuBox, that's 53% above the weight division average of 19/58/33%. That's pretty sick.
I really don't know how Pacquiao will respond to Mayweather's counterpunching threat. He'll either be afraid of getting popped and therefore limit his punch output to avoid getting exposed...or he'll come in with the mindset that "I'm too fast and he can't hurt me" and he'll throw like a madman.
I think he finds a happy medium. Pacquiao was a straight-up brawler in his early career and his only gameplan was commit to volume. As he's moved up and fought men much stronger than him, the chance of getting seriously hurt out of nowhere increased exponentially. With that added threat, Pacquiao's gameplan has evolved. He became craftier, throwing at more confusing angles and showing far more discipline in punch and combo selection, but still retained the same speed and power. I'd call this "smart volume." He'll be very close to that ~70 punch per round average again.
He doesn't need to change anything, he just has to do what he does best. He's fast and powerful and Mayweather's never seen anyone as good as him, just like Pac's never had to hit something as elusive as PBF. Just commit to what brought you to this point, what's made you the best P4P boxer of the last decade, and hope for the best.