In the Twelve Labors of Hercules, ten labors were originally assigned before the traditional total became twelve.
Eurystheus initially required Hercules to complete ten tasks as penance. Two were later disallowed: the Hydra because Hercules received help from Iolaus, and the Augean stables because Hercules used the rivers and expected payment from King Augeas.
Because those two feats were rejected, Eurystheus added two more tasks. The revised sequence therefore contained twelve labors, ending with the capture of Cerberus. This explains why some summaries describe Hercules as completing ten labors while the standard cycle is called the Twelve Labors.
The exact presentation can vary between ancient authors and later retellings. The ten-to-twelve explanation is the conventional account used in modern references and is the key to understanding the numbering.