C. F. Keil and Franz Delitzsch on Numbers 25:9

Num 25:9

Twenty-four thousand men were killed by this plague. The Apostle Paul deviates from this statement in 1Co 10:8, and gives the number of those that fell as twenty-three thousand, probably from a traditional interpretation of the schools of the scribes, according to which a thousand were deducted from the twenty-four thousand who perished, as being the number of those who were hanged by the judges, so that only twenty-three thousand would be killed by the plague; and it is to these alone that Paul refers.