How many rounds are in a standard professional championship boxing match?

How to get close to this number

Bound it by endurance. A round is three minutes with a minute's rest, so a title fight has to fit inside what a boxer can sustain and a broadcast can schedule. Somewhere around an hour of elapsed time is the practical maximum, which puts the round count in the low teens.

The change in the explanation is the important part: the standard was fifteen rounds until the mid-1980s. The reduction followed deaths from injuries sustained in the late rounds, when fighters are most exhausted and least able to protect themselves. Safety rules in combat sports are almost always written after a specific death, and this one was no exception.

That makes the number a policy artefact rather than a property of the sport. Records set under the old distance are not directly comparable, and the last three rounds of a fifteen-round bout were exactly where the sport's most famous late finishes happened. Changing a rule for safety also changes what the sport is capable of producing.