Paul, alone, receives the strong approbation of Peter and obtains Peter's clear emphasis that Paul's writings are 'scripture :
... even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. [2 Peter 3:15,16 KJV]
Matthias was chosen by lot to fill the gap left by the treachery of Judas, Acts 1:20-26, and Matthias was 'numbered with the eleven apostles'. However we then hear nothing more of him, whatsoever.
James, the Lord's brother, appears to have gained status and Paul references this, but without a clear commendation, merely accepting what others had allowed of :
But other of the apostles saw I none, save James the Lord's brother. [Galatians 1:19 KJV]
However, it becomes clear that 'certain' coming from James had been the cause of legal dissension such that even Barnabas had been almost carried away with it.
For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision. [Galatians 2:12 KJV]
But not Paul, who even withstood the chief apostle, given the keys by Jesus Christ himself, withstanding Peter to his face for 'he was to be blamed', Galatians 2:11.
And Barnabas himself, called 'an apostle' along with Paul :
Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out, [Acts 14:14 KJV] . . . .
. . . yet had strife with Paul regarding John Mark after which no more is heard of Barnabas.
Thus, it seems that Matthias, James and Barnabas were all associated with or 'numbered with' the apostles yet did not serve with distinction and only Paul receives an approbation of the highest order such that Peter, the chief apostle, testifies of the former Pharisee's writings that they are to be considered 'scripture'.
In John's visions, recorded in the Apocalypse, there is a definite number attached to the apostles, twelve and no more :
And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb. [Revelation 21:14 KJV]
But we also see Luke and Mark writing their gospel accounts, very much under the influence of both Peter and Paul. So although, perhaps, not numbered as 'of the twelve' yet their spiritual gift and spiritual insight elevates them to high office and respect, Paul saying of Mark that 'he is profitable to me for the ministry', 2 Timothy 4:11, and of Luke, 'the beloved physician', Colossians 4:14, whilst Peter calls Mark 'my son', 1 Peter 5:13.
What does Reformed Protestantism see in the fact of twelve, and only twelve, being the foundation of the city, and yet also the fact that three others were associated, or 'numbered with' the apostles, but not receiving the kind of approbation which Peter offered, only, to Paul ?