Why do you say there were just twelve apostles? It seems to me that there were at least fifteen (see Acts 1:25, 14:14; Gal. 1:19).
In Matthew 10 we have the record of Jesus choosing His apostles. It reads: “Now the name of the twelve apostles are these…” (Matt. 10:2). Luke’s parallel account of this passage also affirms the naming of twelve apostles (Luke 6:13). Frequently, throughout the Gospels, these men are called “the apostles” (Mark 6:30; Luke 9:10; Luke 11:49; 17:5; 24:10) or simply “the twelve” (Matt. 26:14, 20, 47; Mark 4:10; 6:7; 9:35; 10:32; 11:11; Luke 8:1; 9:12; 18:31). After Judas defected, they were called “the eleven” (Matt. 28:16; Mark 16:14; Luke 24:9, 33). These twelve apostles (or later the eleven) were especially chosen by Jesus, and sent out to be His messengers and witnesses bearing His message.
It is also clear that Judas by transgression forfeited his privileged position, as was prophesied (Ps. 109:8), “Let another take his office” (Acts 1:20).
Jesus’ command to His disciples just before His ascension, as recorded in Luke 24:49 and Acts 1:4, was, “behold, I send the promise of my Father upon you: but tarry in the city of Jerusalem, until you are endued with power from on high.” They were told to tarry, and to “wait for the promise of the Father,” the Holy Spirit, which they should receive “not many days hence” (Acts 1:5).
It seems that Peter, despite the definite command to “wait,” overstepped his authority and took it upon himself to initiate official business which he was not commissioned to have undertaken. The apostles sought guidance through prayer, cast lots, and Matthias was chosen. We are not told directly whether this action, initiated by Peter, was right or wrong, but we do not hear anymore of Matthias after this mentioning. Matthias may have been a faithful brother and follower of Christ, however evidence does not show that he was properly appointed to the special office of an apostle.
Evidence does show that Jesus Himself appointed Paul to be an apostle. In Acts 22:14-15 we have Paul’s own account of his conversion and call to the apostleship, along with his qualifications.
After Paul’s dramatic meeting with Jesus, when he was blinded, Ananias was instructed in a divinely inspired vision to open Paul’s eyes and to reveal what his work was to be in these words: “Go, for he is a chosen vessel of Mine, to bear My name before the Gentiles, and kings, and the children of Israel” (Acts 9:15).
Numerous times in his Epistles he affirms his appointment as an apostle. Nine of his Epistles open with this salutation: “Paul an apostle of Jesus Christ …”
Additional evidence is that in the Revelator’s vision, where the Holy City, symbolic of Christ’s faithful bride or church, had a wall which had “twelve foundations, and in them the names of the twelve apostles of the Lamb;” (Rev. 21:14) twelve, no more and no less.
If there were only twelve apostles, why is Barnabas called an apostle? (Acts 14:14)? Or why is “James the Lord’s brother” called an apostle (Gal. 1:19)?
The definition of apostolos in the Greek suggests that the term was used “specifically of the twelve apostles of Christ,” but also “in a broader sense applied to other eminent Christian teachers” (Enhanced Strong’s Lexicon). Both Barnabas and James qualified as eminent Christian teachers, though they were not of the original chosen twelve. In this broader sense, any disciple could be an apostle, though technically the definition is still limited, as Paul said, to those who had seen “Jesus Christ our Lord” (1 Cor. 9:1).