Objection 1. It would seem that suffrages offered for several are of as much value to each one as if they had been offered for each in particular. For it is clear that if one person receives a lesson he loses nothing if others receive the lesson with him. Therefore in like manner a person for whom a suffrage is offered loses nothing if some one else is reckoned together with him: and consequently if it be offered for several, it is of as much value to each one as if it were offered for each in particular.
Objection 2. Further, it is to be observed that according to the common practice of the Church, when Mass is said for one deceased person, other prayers are added for other deceased persons. Now this would not be done, if the dead person for whom the Mass is said were to lose something thereby. Therefore the same conclusion follows as above.
Objection 3. Further, suffrages, especially of prayers, rely on the Divine power. But with God, just as it makes no difference whether He helps by means of many or by means of a few, so it differs not whether He assists many or a few. Therefore if the one same prayer be said for many, each one of them will receive as much assistance as one person would if that same prayer were said for him alone.
On the contrary, It is better to assist many than one. If therefore a suffrage offered for several is of as much value to each one as if it were offered for one alone, it would seem that the Church ought not to have appointed a Mass and prayer to be said for one person in particular, but that Mass ought always to be said for all the faithful departed: and this is evidently false.
Further, a suffrage has a finite efficiency. Therefore if it be divided among many it avails less for each one than if it were offered for one only.
I answer that, If the value of suffrages be considered according as it is derived from the virtue of charity uniting the members of the Church together, suffrages offered for several persons avail each one as much as if they were offered for one alone, because charity is not diminished if its effect be divided among many, in fact rather is it increased; and in like manner joy increases through being shared by many, as Augustine says (Confess. viii). Consequently many in purgatory rejoice in one good deed no less than one does. On the other hand, if we consider the value of suffrages, inasmuch as they are a kind of satisfaction applied to the dead by the intention of the person offering them, then the suffrage for some person in particular avails him more than that which is offered for him in common with many others; for in this case the effect of the suffrages is divided in virtue of Divine justice among those for whom the suffrages are offered. Hence it is evident that this question depends on the first; and, moreover, it is made clear why special suffrages are appointed to be offered in the Church.
Reply to Objection 1. Suffrages considered as works of satisfaction do not profit after the manner of an action as teaching does; for teaching, like any other action, produces its effect according to the disposition of the recipient. But they profit after the manner of the payment of a debt, as stated above (12, ad 1); and so the comparison fails.
Reply to Objection 2. Since suffrages offered for one person avail others in a certain way, as stated (1), it follows that when Mass is said for one person, it is not unfitting for prayers to be said for others also. For these prayers are said, not that the satisfaction offered by one suffrage be applied to those others chiefly, but that the prayer offered for them in particular may profit them also.
Reply to Objection 3. Prayer may be considered both on the part of the one who prays, and on the part of the person prayed: and its effect depends on both. Consequently though it is no more difficult to the Divine power to absolve many than to absolve one, nevertheless the prayer of one who prays thus is not as satisfactory for many as for one.
Source: Summa Theologica (New Advent)