I assume you're only talking about champion fighters? There are tons of lesser-known guys who had to work as sparring partners extensively to put food on the table.
Philly guys used to be notorious for their over-the-top gym wars. Guys in the Briscoe, Hart, Watts era. Meldrick Taylor too.
On a BBC podcast the broadcaster mentioned that Amir Khan sparred 250 rounds just for the Algieri bout.
Manny Steward mentioned Chavez didn't do any pad work until he trained with him, all of his subtle tricks were learned in the ring, Steward said Chavez taught the legendary trainer lots.
The very first article I read about Chavez in 86 maybe made reference to him doing mitt work in the :.