[Rev 21:10-14 ASV] (10) And he carried me away in the Spirit to a mountain great and high, and showed me the holy city Jerusalem, coming down out of heaven from God, (11) having the glory of God: her light was like unto a stone most precious, as it were a jasper stone, clear as crystal: (12) having a wall great and high; having twelve gates, and at the gates twelve angels; and names written thereon, which are the names of the twelve tribes of the children of Israel: (13) on the east were three gates; and on the north three gates; and on the south three gates; and on the west three gates. (14) And the wall of the city had twelve foundations, and on them twelve names of the twelve apostles of the Lamb.
Why are there only 12 foundation stones? Why does not Paul get a foundation stone?
First, we do not know that he did not. However, the fact that he is usually not counted among the original 12 is presumably what the question is asking. But neither is Barnabas (Acts 14:14), nor Silas(1 Thess 1:1, 2:6), both of whom were indisputably apostles. But there are many more such as Timothy (1 Thess 1:1, 2:6), Andronicus and Junia (Rom 16:7) and Apollos (1 Cor 4:9). I am sure there were many more then and since.
The book of Revelation uses the number 12 as a kind of symbol of the Kingdom with its 12 tribes made up of 12 thousand people (Rev 7) and the city walls 12 thousand furlongs long, etc. Note that these 12 foundation stones are reminiscent pf the 12 stones on the breastplate of the OT High Priest which the 12 apostles represented (as Jesus) when they were sent out. Thus, anyone who is an apostle of Christ represents the great High Priest, Jesus (Heb 4:14-16, 10:21).
Therefore, I suspect, 12 is simply proverbial in a sense of the original apostles' teaching that even Paul checked with before beginning his ministry (Gal 2).
By way of comparison, the passage speaks of "the names of the twelve tribes of the children of Israel" without naming them. Presumably John knows the names and that there different ways the twelve tribes may be listed. Thus the silence on the specific names is intentional. Perhaps this is purposeful to avoid the questions which will arise about names not on the list. In other words, by speaking generally of the "twelve tribes" the reader is to focus on the inclusion of all and avoid speculating on the possible exclusion of some.
Nevertheless, there is a difference between the twelve tribes and the twelve apostles. Because the names of the tribes are associated with gates, the symbolism is every tribe may enter. The names of the apostles, on the other hand, are associated with the foundations of the wall, which suggest keeping out, or at least making a distinction between inside and outside.
It is probably significant that Jesus never specifically identifies "apostles." There are only two passages He is recorded using the word:
Therefore also the Wisdom of God said, ‘I will send them prophets and apostles, some of whom they will kill and persecute,’ (Luke 11:49) [ESV]
“‘I know your works, your toil and your patient endurance, and how you cannot bear with those who are evil, but have tested those who call themselves apostles and are not, and found them to be false. (Revelation 2:2)
Both times the implication is there are more than twelve. In support of this, the three uses of the word in Revelation appear to be arranged as a chiastically:
A: I know your works, your toil and your patient endurance, and how you cannot bear with those who are evil, but have tested those who call themselves apostles and are not, and found them to be false. (Revelation 2:2) X: Rejoice over her, O heaven, and you saints and apostles and prophets, for God has given judgment for you against her!” (Revelation 18:20) A': And the wall of the city had twelve foundations, and on them were the twelve names of the twelve apostles of the Lamb. (Revelation 21:14)
The main point at 18:20 allows for more than twelve: all who are apostles will rejoice. The logic connecting A and A' is some are not apostles which parallels the necessity of excluding some to arrive at twelve.
The use in 2:2 is directed to those in Ephesus, but since it is coming directly from Jesus, it would not be inappropriate to apply the logic to the original twelve. Note in 2:2 the ending emphasis is on false apostles implying the foundations are built on true apostles. If one wanted to speculate on specific names, Paul would be the logical replacement for Judas, the likely omission from the original twelve.
However, the failure to list the twelve names must be taken as intentional and like the failure to specify which names comprise the twelve tribes, purposeful to avoid speculation on who is, or is not included. The necessity of a wall simply requires twelve true apostles of the Lamb to serve as foundations. Any who are called apostles but not one of the twelve would still be included with those who rejoice (18:20) and as all were of the twelve tribes, eligible to enter.
We mustn't take for granted that Paul wont be among the foundation stones in the glorious heavenly city; the New Jerusalem. It is probably better to ask: "Is Paul one of the living stones making up the foundation of Christianity, and the New Heavenly Jerusalem"? It would be very strange if he wasn't included, considering his wisdom in salvational matters. Much of his wisdom is original and can not easily be seen elsewhere. His focus on the cross of Christ and the crucifixion of the flesh is foundational Christian teaching.
Gal 6:14 (NIV) May I never boast except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world.
1 Cor 1:18 (NIV) For the message of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God.
Phil 3:18 (NIV) For, as I have often told you before and now tell you again even with tears, many live as enemies of the cross of Christ.