Was Paul or Matthias the 12th apostle? This is a curious debate that is very important to some folks. Whose name will be on the foundation of the walls of the holy city (Revelation 21:14)? This debate raises the question of whether Matthias was legitimately chosen, or if Paul was God's intended "twelfth apostle".
The selection of Matthias, Acts 1:15-26
And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty,) Men and brethren, this scripture must needs have been fulfilled, which the Holy Ghost by the mouth of David spake before concerning Judas, which was guide to them that took Jesus. For he was numbered with us, and had obtained part of this ministry. Now this man purchased a field with the reward of iniquity; and falling headlong, he burst asunder in the midst, and all his bowels gushed out. And it was known unto all the dwellers at Jerusalem; insomuch as that field is called in their proper tongue, Aceldama, that is to say, The field of blood. For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his bishoprick let another take. Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us, Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection. And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias. And they prayed, and said, Thou, Lord, which knowest the hearts of all men, shew whether of these two thou hast chosen, That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place. And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.
The apostles had the Spirit. They had been instructed by Christ. They had the Scriptures. The writer Luke, writing under the inspiration of the Holy Spirit, never gives the least hint that anything is amiss. The 120 disciples were all in accord both before and after the selection of Matthias, and God poured out His Spirit on these united disciples. Matthias was numbered with the twelve, and is so numbered by divine inspiration by Luke in Acts and by Paul in 1 Corinthians 15.
One thing that could make this debate look almost silly is the fact that at least 19 men, and possibly as many as 23, are called apostles in the New Testament. Both Matthias and Paul were legitimate apostles, as were Barnabas and James. This is not an "either/or" question.
Some have pointed out that the term “the twelve” is used as a collective term -- that is, whether there were always twelve or not (whether existing or present), the apostles as a group were designated “the twelve.” That is correct, though the anti-Matthias crowd often ignore that when convenient. John 20:24 is a good example of “the twelve” used as a collective designation. On one occasion after the resurrection “...Thomas, one of the twelve, called Didymus, was not with them when Jesus came.” Even though there were actually only ten disciples at that time—since Thomas was not with them and Judas was dead—they were still “the twelve.” In contrast, some of the "anti-Matthias" order point out that 1 Corinthians 15:5 and 8 are “clearly sequential” -- that is, only dealing with the chronological a sequence of events and do not speak to Matthias' apostleship. In order of time, Jesus He appeared to “the twelve” and then later in time to Paul. What must be noticed, though, is that Paul clearly does not include himself in this “collective term” either after the resurrection or at the time he is writing.
Paul did not assume the apostleship or office of Judas Iscariot, but had his own unique apostleship. Rather than get from the text that there is something defective in Matthias' apostleship, we must start with that assumption and read it into the text. This view rests more on assumption than Scriptural evidence. The fact of the matter is that Matthias was chosen, not Paul, and no hint to the contrary is ever given.