"The Twelve" (includes Matthias) were for (genetic) Israel.
Paul was the Apostle for the Gentiles.
Rom 11:13 For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, - witness #1
I Tim 2:7, ...I am ordained a preacher, and an apostle, .... and a teacher of the Gentiles... - witness #2
2 Tim 1:11 Whereunto I am appointed a preacher, and an apostle, and a teacher of the Gentiles. - witness #3
II Cor 13:1
Deut 19:15
In the mouth of two or three witnesses shall every word be established
So, if you are a Gentile, then Paul is your Apostle ...not Peter, not John, not James.
So there are 13 apostles?
Revelation 21:14 And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb.