Adam Clarke on Numbers 25:9

Those that died - were twenty and four thousand - St. Paul, 1Cor 10:8, reckons only twenty-three thousand; though some MSS. and versions, particularly the latter Syriac and the Armenian, have twenty-four thousand, with the Hebrew text. Allowing the 24,000 to be the genuine reading, and none of the Hebrew MSS. exhibit any various reading here, the two places may be reconciled thus: 1,000 men were slain in consequence of the examination instituted Num 25:4, and 23,000 in consequence of the orders given Num 25:5; making 24,000 in the whole. St. Paul probably refers only to the latter number.