Another thing we have to keep in mind is that a single Mass is of infinite value. There's technically no difference between having one Mass or 1,000 Masses offered for the same intention, as the single Mass could have the same benefit as 1,000.
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Hmm, interesting point. I've seen this question discussed in books on the Mass, and everything I've read says the opposite. The explanation is a bit complex. Yes, the Mass is infinite with regard to God, but finite with regard to man, because the person who offers it does so with only a certain amount of fervor and devotion, likewise the priest who says the Mass, etc.
The Mass accomplishes the four ends of prayer: adoration, contrition, thanksgiving and supplication. Since there is no intrinsic limit on the amount of adoration or thanksgiving we can give to God, the books say that every Mass gives an infinite amount of those two things to God, but contrition refers to having our sins or temporal punishment forgiven, and supplication refers to what favors we can obtain from God. Since those two things are finite, the amount of temporal punishment the Mass can remove is limited by our dispositions, as well as the amount of favors we can ask for.
This is why the Church encourages people to have many Masses offered for an intention, and why she has supported things like foundations of Masses, in which some rich nobleman would leave a fortune to a monastery when he dies, in exchange for them saying Mass for him every day until the end of the world. If the effect of one Mass were the same in this sense as 1,000 Masses, such an act would be meaningless.
Also, the authors say that if one Mass had infinite benefits with regard to the human race through all four ends of prayer, including contrition and supplication, one Mass would empty out Purgatory and convert every sinner in the world and obtain every good thing possible for every human being, which is clearly not the case.