Well, Scripture makes it abundantly clear that Paul is an apostle (see Rom 1:1, Rom 11:13, 1 Cor 1:1-2, 1 Cor 15:9, 2 Cor 1:1, Gal 1:1, Eph 1:1, Col 1:1, 1 Tim 1:1, 1 Tim 2:7, 2 Tim 1:1, 2 Tim 1:11, Titus 1:1).
In fact, Scripture also names the following as apostles: Epaphroditus (Phil 2:25), Barnabas (Acts 14:13), Andronicus and Junias (Rom 16:7).
We also see Peter referred to as an apostle: 1 Pet 1:1, 2 Pet 1:1, but also referenced separately from the apostles: 1 Cor 9:5.
Actually, even Jesus is called an apostle: Heb 3:1, Eph 2:20.
Paul also indicates that his apostleship isn’t a position or office, but is a calling, as he says he is an apostle to some, but not to others (1 Cor 9:2), and often refers to his apostleship as * to the Gentiles* (Rom 11:13, 2 Tim 1:11, 1 Tim 2:7), whereas others have apostleship of the circumcision (Gal 2:8).
This makes more sense when you understand the word “apostle” comes from the Greek “apostolos” which means “messenger, person sent forth.” (etymonline.com/index.php?allowed_in_frame=0&search=apostle&searchmode=none).
The apostles were emissaries, and this calling is derived from the Great Commission given by Christ to go out and teach all nations. He is the chief apostle because He alone brought the Gospel to the world, and inasmuch as we act as apostles, emissaries, we act * in Christ*, as in all things.
And as to this calling, it is often implied in Scripture that apostleship is a calling, one among many, that belongs to many Christians according to God’s choosing (1 Cor 12:28-29, Eph 4:11). Indeed, this implication is reinforced by the talk of false apostles (2 Cor 11:13, Rev 2:2), which wouldn’t make sense if there were only the twelve.
But what about the Twelve? These are the Twelve Apostles who were called by Jesus during His ministry, and Matthias who was elected after the betrayal of Judas. This is a distinct group. After Jesus, they were the first apostles (emissaries) of Jesus, and as the foundation apostles (Rev 21:14), they had to have been eyewitnesses to all that Jesus said and did during His ministry (Acts 1:21-22) in order to fulfill the Great Commission through oral tradition (and letters).
These twelve are a unique group and are:
sometimes referred to as “the twelve apostles” (Mat 10:2, Luke 9:1, Luke 22:14, Rev 21:14);
sometimes referred to as “the twelve disciples” (Mat 20:17, Mat 10:1, Mat 11:1, Mat 26:20, Luke 6:13);
and very often referred to as simply “the twelve” (Mat 26:14, Mat 26:47, Mark 4:10, Mark 6:7, Mark 9:34, Mark 10:32, Mark 11:11, Mark 14:10, Mark 14:17, Mark 14:20, Mark 14:43, Luke 8:1, Luke 9:12, Luke 18:31, Luke 22:3, Luke 22:47, John 6:68, John 6:72, John 20:24, Acts 6:2, James 1:1)
Of course, this foundation on “the twelve” is in direct reference to the foundation of the Church as the new Israel, as the old Israel was founded upon twelve tribes or patriarchs (Gen 25:16, Gen 49:28, Exo 24:4, Exo 28:21, Exo 39:14, Ecc 44:26, Eze 47:13, Jos 3:12, Numb 17:2, Mat 19:28, Luke 22:30, Acts 26:7, James 1:1, Rev 21:12, Acts 7:8).
Indeed, we see a direct correlation between the twelve tribes and the twelve apostles as Israel in the following two passages:
1 Kings 18:31 - And he took **twelve stones **according to the number of the tribes of the sons of Jacob, to whom the word of the Lord came, saying: Israel shall be thy name.
Rev 21:14-21 - And** the wall of the city had twelve foundations**, and in them, the twelve names of the twelve apostles of the Lamb…] And the **foundations of the wall of the city were adorned with all manner of precious stones. **The first foundation was jasper: the second, sapphire: the third, a chalcedony: the fourth, an emerald:  The fifth, sardonyx: the sixth, sardius: the seventh, chrysolite: the eighth, beryl: the ninth, a topaz: the tenth, a chrysoprasus: the eleventh, a jacinth: the twelfth, an amethyst. And the twelve gates are twelve pearls, one to each: and every several gate was of one several pearl.
It is clear from Scripture that Paul was never counted as one of the Twelve, though Matthias was. Read Rev 21:21 again. Note: “every several gate” and “one several pearl.” This is a mystical account. There is a plurality in unity spoken of here. That is to say, the twelve are the foundation of the Church, and whatever sharing in the callings and graces of the Church that we receive from Christ (such as apostleship) comes to us through the Twelve, the foundation of the Church, where Christ is the Chief Cornerstone.
Thus, Paul and Epaphroditus and Barnabas (who had been considered for the position among the Twelve, but was not counted as such) and Andronicus and Junias are all apostles, but they are not counted among the Twelve.