Forty: 2 Paralipomenon ix. 25., has four in the Hebrew. Septuagint read in both places 40,000 mares, for chariots, and 12,000 horses. (Calmet)

The Alexandrian copy has 40 here, and 4000 in the latter place; where, instead of horses, it gives horsemen, with the Vulgate. These two words are often used as synonymous by the best authors. But it is more difficult to reconcile the number; (Calmet) as (2 Paralipomenon xiv.) we read again differently, he had 1400 chariots, and 12,000 horsemen. (Haydock)

Forty might easily be mistaken for four, by only adding im at the end of arba. (Bochart) (Grotius)

Instead of stalls, Calmet supposes stables to be understood, and in each he would place ten horses, which completes the number here assigned. If this be admitted, no change is necessary: but, as præsepe signifies “a stall,” we may adhere to the Vulgate, which has 40,000 in both places; whereas the Hebrew varies, though the sense may be the same. The number of Solomon’s chariots was 1400. As two horses were usually employed to draw them, 2800; or, allowing for accidents, changes, &c., 4000 horses would have been amply sufficient. It seems, therefore, that we should admit only so many horses or stalls. (Haydock)

“Vignoles conjectures, that the Jews formerly used marks analogous to our common figures; as the Arabians have done for many hundred years. And, if so, the corruption” of hundreds for tens, &c., “may be easily accounted for, by the transcriber’s carelessly adding or omitting a single cypher.“ (Kennicott, Diss. ii.)

Yet, if 40,000 horses must be admitted, we may say that they were not all intended for the chariots of war, but some for draught-horses, to convey the stones and other materials for the numerous buildings, which Solomon carried on. This might serve to excuse him for having so many horses, (Haydock) contrary to the letter of the law, and the example of Josue and of David. His subjects were thus, perhaps, engaged in too much commerce with the Egyptians: and the king was forced to burden them with taxes, which at last proved so fatal. (Serarius) (Pineda) (Calmet)

Yet some undertake his defence, by saying that he did not act against the spirit of the law; that many of the horses were imposed as a tribute, and Solomon did not place his trust in them, Proverbs xxi. 31., and 2 Paralipomenon ix. 24. (Tostat) (Bochart, B. ii. 9.)

His empire was become more extensive, and his works more splendid; so that what might appear an useless parade in some, might be worthy of praise in Solomon. The law is not so precise. He shall not multiply horses to himself, nor lead back the people into Egypt, being lifted up with the number of his horsemen, Deuteronomy xvii. 16. There is a like prohibition of many wives and treasures.