St Matthias was of course the apostle elected to replace Judas. The readings for the second Nocturn, by St Augustine, go to the importance of having twelve apostles:
Reading 5: Her foundation is in the holy mountains the Lord loveth the gates of Zion. Wherefore hath the city twelve foundations, and in them the names of the Prophets and of the Apostles of the Lamb? Because their authority is the foundation whereon our weakness resteth. Wherefore are they the gates? Because through them we enter in unto the kingdom of God, since they have preached the same unto us, and when we enter in through their preaching, we enter in by Christ, Who is Himself The Door. John x. 7. And, whereas it is written that the city hath twelve gates, and, again, that Christ is the one Door, Christ is all the twelve, for He is in all the twelve and therefore were twelve Apostles chosen.
Reading 6: There lieth a great mystery in the signification of this number twelve: Ye shall sit, said the Lord upon twelve thrones, judging the twelve tribes of Israel.If then there be set there twelve thrones of judgment, Ps. cxxi. 5, Paul, in that he is the thirteenth Apostle, hath not where to sit, nor wherein to judge. Nevertheless, he hath said of himself that he will judge not men only, but angels. Know ye not, saith he, that we shall judge angels? i Cor. vi. 3, that is, the fallen angels. Then might they have answered him Wherefore boastest thou thyself to be a judge? For where is thy seat? The Lord hath said that for the twelve Apostles there shall be twelve thrones one of the twelve, even Judas, is indeed fallen, but holy Matthias is chosen into his place; for the twelve thrones there are still twelve to sit thereon first find whereon thou shalt sit, and afterward give thyself out for a judge.
Reading 7: Let us see, then, what is the meaning of these twelve thrones. By them is signified in a mystery the whole world, since the Church shall be through all the earth, whence this building is called to be built up together in Christ.Therefore is it said that there shall be twelve thrones, because from all quarters shall there come men to be judged; even as it is said that the city hath twelve gates, because from all quarters shall the nations of them which are saved, enter into it.
Reading 8: So, not the twelve only, and the Apostle Paul, but all, as many as shall judge, have part in these twelve thrones, this signifying, that they shall judge all men; even as all that enter into the city, have part in her twelve gates. For there are four quarters of the world, the East, and the West, and the North, and the South of which four quarters is mention often made in the Scriptures. From the four winds shall the elect be gathered together, as saith the Lord in the Gospel And He shall send His Angels with a great sound of a trumpet; and they shall gather together His elect from the four winds, from one end of heaven to the other. Matth. xxiv. 31. From the four winds, therefore, is the Church called together; and how are they called? Everywhere are they called in the Trinity; for they are called no otherwise than by baptizing them in the Name of the Father, and of the Son, and of the Holy Ghost. Matth. xxvii. 19. Now four being multiplied by three is twelve.