Objection 1. It would seem that the Old Law contains but one precept. Because a law is nothing else than a precept, as stated above (90, A2,3). Now there is but one Old Law. Therefore it contains but one precept.
Objection 2. Further, the Apostle says (Romans 13:9): "If there be any other commandment, it is comprised in this word: Thou shalt love thy neighbor as thyself." But this is only one commandment. Therefore the Old Law contained but one commandment.
Objection 3. Further, it is written (Matthew 7:12): "All things... whatsoever you would that men should do to you, do you also to them. For this is the Law and the prophets." But the whole of the Old Law is comprised in the Law and the prophets. Therefore the whole of the Old Law contains but one commandment.
On the contrary, The Apostle says (Ephesians 2:15): "Making void the Law of commandments contained in decrees": where he is referring to the Old Law, as the gloss comments, on the passage. Therefore the Old Law comprises many commandments.
I answer that, Since a precept of law is binding, it is about something which must be done: and, that a thing must be done, arises from the necessity of some end. Hence it is evident that a precept implies, in its very idea, relation to an end, in so far as a thing is commanded as being necessary or expedient to an end. Now many things may happen to be necessary or expedient to an end; and, accordingly, precepts may be given about various things as being ordained to one end. Consequently we must say that all the precepts of the Old Law are one in respect of their relation to one end: and yet they are many in respect of the diversity of those things that are ordained to that end.
Reply to Objection 1. The Old Law is said to be one as being ordained to one end: yet it comprises various precepts, according to the diversity of the things which it directs to the end. Thus also the art of building is one according to the unity of its end, because it aims at the building of a house: and yet it contains various rules, according to the variety of acts ordained thereto.
Reply to Objection 2. As the Apostle says (1 Timothy 1:5), "the end of the commandment is charity"; since every law aims at establishing friendship, either between man and man, or between man and God. Wherefore the whole Law is comprised in this one commandment, "Thou shalt love thy neighbor as thyself," as expressing the end of all commandments: because love of one's neighbor includes love of God, when we love our neighbor for God's sake. Hence the Apostle put this commandment in place of the two which are about the love of God and of one's neighbor, and of which Our Lord said (Matthew 22:40): "On these two commandments dependeth the whole Law and the prophets."
Reply to Objection 3. As stated in Ethic. ix, 8, "friendship towards another arises from friendship towards oneself," in so far as man looks on another as on himself. Hence when it is said, "All things whatsoever you would that men should do to you, do you also to them," this is an explanation of the rule of neighborly love contained implicitly in the words, "Thou shalt love thy neighbor as thyself": so that it is an explanation of this commandment.
Source: Summa Theologica (New Advent)