Hear the word "Apostles" and immediately the mind thinks of the contradictory lists of 12 men in the Gospels. However, the word (apóstolos) had a much broader usage in the Greek speaking world, pagan, Jewish and Christian. For simplicity I'd copy a few definitions.
The word apóstolos occurs only in the LXX in 1 Kings 14:6, where it is used in a non-technical sense to represent the Heb. שָׁל֥וּחַ, from the verb שָׁלַח, H8938, “to send.” The idea of God sending His servants the prophets is frequent in the OT, and the verb is normally translated by apostéllō in the LXX. The noun שָׁלִיחַ is used of Moses, Elijah, Elisha, and Ezekiel. In later Judaism this word was used to refer to those who acted as representatives for others (“a man’s shalīaḥ is as himself”). In particular it referred to accredited agents, often sent out in pairs, going from the authorities in Jerusalem to the Diaspora.....Christ Himself is described as apóstolos in Hebrews 3:1....It is used also to refer to messengers from the churches on two occasions (2 Cor 8:23; Phil 2:25). There is also an interesting use of the word to describe God’s messengers to Israel (Luke 11:49).Apostle - Encyclopedia of The Bible - Bible Gateway
The writer of some Paulines refers to traveling companions of Paul as 'apostles' (1 Thess 2:6, 1 Cor 4:9 etc),
6 Nor did we seek glory from people, whether from you or from others, though we could have made demands as apostles of Christ.
He uses the term superfine-apostles derisively speaking of those who he saw as arrogant but highly honored as teachers. (2 Cor 11:5)
Even regarding a woman as among, and even "prominent among, the Apostles' (Romans 16:7)
Yes translators have creatively found ways to render this passage so as to exclude Junia from the apostles, but they are stretching the simplest and clearest reading, the one shared by early prominent writers such as Origen:
Greet Andronicus and Junia, my kinsmen and fellow prisoners, who are notable among the apostles, who also were in Christ before me. It can certainly be said both that they were kinsmen of Paul according to the flesh, and had believed before him, and were held as notable among the apostles of Christ; about whom (it is also possible that it should be understood in this way, that perhaps they were from those seventy-two who are also themselves named apostles,) and therefore he calls them notable among the apostles, even those apostles who were before him.
Also Chrysostom:
And indeed, to be apostles at all is a great thing. But to be amongst these of note, just consider what a great encomium this is! But they were of note owing to their works, to their achievements. Oh! How great is the devotion of this woman, that she should be even counted worthy of the appellation of apostle.
Note that the 'appellation of apostle' was an honorific one. Origen recalls the symbolic 70/72 'apostles' in Luke 9 (which is an expansion of the story in Mark 6 referencing the 70 nations in Genesis). Origen's (and Eastern Church today) assumption the word 'apostle' applied to the story in Luke (though not present now) as the word had not become intimately/exclusively associated with the 12 of the Gospels yet. This is transparent in the Didache.
The Didache (first century) chapt 11:
3Now about the apostles and prophets: Act in line with the gospel precept. [519] ?^4Welcome every apostle on arriving, as if he were the Lord. ?^5But he must not stay beyond one day. In case of necessity, however, the next day too. If he stays three days, he is a false prophet. ?^6On departing, an apostle must not accept anything save sufficient food to carry him till his next lodging. If he asks for money, he is a false prophet.
My above summary is for a purpose. It is my view that far too often when readers hear expressions like "teaching of the apostles" we assume it a reference to 12 quasihistorical guys. That is not necessarily the case. It appears from the scant materials we have surviving that the expression was an honorific appellation credited due to special assignments as emissaries of church leadership or more broadly of Christ. It seems in come contexts to have been associated with prophets (like in the OT). It was in some cases self-declared, such as with Paul.
There is much more that can be said.