9 But what the Lord did after His resurrection, signified what is to be to us after our resurrection, in that number of the kingdom of heaven, where shall be no bad man....Lastly, those seven thousand of whom reply was made to Elias, “I have left me seven thousand men that have not bowed knees before Baal,” far exceed that number of fishes. Therefore the hundred and fifty-three fishes does not alone express just such a number of saints, but Scripture does express the whole number of saints and righteous men by so great a number for a particular reason; to wit, in order that in those hundred and fifty-three all may be understood that pertain to the resurrection to eternal life.
For the Law has ten commandments: but the Spirit of Grace, through which alone the Law is fulfilled, is called sevenfold. The number then must be examined, what mean ten and seven: ten in commandments, seven in the grace of the Holy Spirit: by which grace the commandments are fulfilled. Ten then and seven contain all that pertain to the resurrection, to the right hand, to the kingdom of heaven, to life eternal, that is, they that fulfil the Law by the Grace of the Spirit, not as it were by their own work or their own merit.
But ten and seven, if you count from one unto seventeen, by adding all the numbers by steps, so that to one you may add two, add three, add four, that they may become ten, by adding five that they may become fifteen, by adding six that they may become twenty-one, by adding seven that they may become twenty-eight, by adding eight that they may become thirty-six, by adding nine that they may become forty-five, by adding ten that they may become fifty-five, by adding eleven that they may become sixty-six, by adding twelve that they may become seventy-eight, by adding thirteen that they may become ninety-one, by adding fourteen that they may become one hundred and five, by adding fifteen that they may become one hundred and twenty, by adding sixteen that they may become one hundred and thirty-six, by adding seventeen, make up one hundred and fifty-three, you will find a vast number of all saints to belong to this number of a few fishes.
In like manner then as in five virgins, countless virgins; as in five brethren of him that was tormented in hell, thousands of the people of the Jews; as in the number of one hundred and fifty-three fishes, thousands of thousands of saints: so in twelve thrones, not twelve men, but great is the number of the perfect.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)