Mickey Lolich in 1971 - 376 innings, 45 starts, 29 complete games and he was a strikeout guy too. That's insane.
Followed that year up with 327, 308, 308. Even at 35, after those 300+ years, he pitched 192 innings and had an ERA of 3.22
Then in 1976 Mark Fidrych had 24 complete games, 3 of them going 11 innings and they broke him.