I recall Fisher beat Spassky 12.5 - 8.5 in 1972.
FIDE changed the rules from best of 24 to “first to win 6” with draws not counting anything in 1975. Fisher refused the new format, and Karpov was declared champion. A 12-12 outcome would have meant Spassky would have retained his title. I’m not sure but Fisher might have gotten a rematch the following year.
Karpov vs. Korchnoi in 1978 went with the new format. As I recall the tournament went on for months. Or maybe that was 1981...or both.
Seems more placid these days.