1 Corinthians 10:8

fornication--literally, Fornication was generally, as in this case (Num. 25:1-18), associated at the idol feasts with spiritual fornication, that is, idolatry. This all applied to the Corinthians (Co1 5:1, Co1 5:9; Co1 6:9, Co1 6:15, Co1 6:18; Co1 8:10). Balaam tempted Israel to both sins with Midian (Rev 2:14). Compare Co1 8:7, Co1 8:9, "stumbling-block," "eat . . . thing offered unto . . . idol." three and twenty thousand--in Num 25:9 "twenty and four thousand." If this were a real discrepancy, it would militate rather against inspiration of the subject matter and thought, than against verbal inspiration. The solution is: Moses in Numbers includes all who died "in the plague"; Paul, all who died "in one day"; one thousand more may have fallen the next day [KITTO, Biblical CyclopÃ&brvbrdia]. Or, the real number may have been between twenty-three thousand and twenty-four thousand, say twenty-three thousand five hundred, or twenty-three thousand six hundred; when writing generally where the exact figures were not needed, one writer might quite veraciously give one of the two round numbers near the exact one, and the other writer the other [BENGEL]. Whichever be the true way of reconciling the seeming discrepant statements, at least the ways given above prove they are not really irreconcilable.

— Jamieson-Fausset-Brown Commentary (public domain)