Objection 1. It seems that the Eucharist is not one sacrament but several, because it is said in the Collect [Postcommunion "pro vivis et defunctis"]: "May the sacraments which we have received purify us, O Lord": and this is said on account of our receiving the Eucharist. Consequently the Eucharist is not one sacrament but several.
Objection 2. Further, it is impossible for genera to be multiplied without the species being multiplied: thus it is impossible for one man to be many animals. But, as stated above (Question 60, Article 1), sign is the genus of sacrament. Since, then, there are more signs than one, to wit, bread and wine, it seems to follow that here must be more sacraments than one.
Objection 3. Further, this sacrament is perfected in the consecration of the matter, as stated above (1, ad 3). But in this sacrament there is a double consecration of the matter. Therefore, it is a twofold sacrament.
On the contrary, The Apostle says (1 Corinthians 10:17): "For we, being many, are one bread, one body, all that partake of one bread": from which it is clear that the Eucharist is the sacrament of the Church's unity. But a sacrament bears the likeness of the reality whereof it is the sacrament. Therefore the Eucharist is one sacrament.
I answer that, As stated in Metaph. v, a thing is said to be one, not only from being indivisible, or continuous, but also when it is complete; thus we speak of one house, and one man. A thing is one in perfection, when it is complete through the presence of all that is needed for its end; as a man is complete by having all the members required for the operation of his soul, and a house by having all the parts needful for dwelling therein. And so this sacrament is said to be one. Because it is ordained for spiritual refreshment, which is conformed to corporeal refreshment. Now there are two things required for corporeal refreshment, namely, food, which is dry sustenance, and drink, which is wet sustenance. Consequently, two things concur for the integrity of this sacrament, to wit, spiritual food and spiritual drink, according to John: "My flesh is meat indeed, and My blood is drink indeed." Therefore, this sacrament is materially many, but formally and perfectively one.
Reply to Objection 1. The same Collect at first employs the plural: "May the sacraments which we have received purify us"; and afterwards the singular number: "May this sacrament of Thine not make us worthy of punishment": so as to show that this sacrament is in a measure several, yet simply one.
Reply to Objection 2. The bread and wine are materially several signs, yet formally and perfectively one, inasmuch as one refreshment is prepared therefrom.
Reply to Objection 3. From the double consecration of the matter no more can be gathered than that the sacrament is several materially, as stated above.
Source: Summa Theologica (New Advent)