There is nothing in the Greek which requires ‘apostle’ to be translated with either a capital or small ‘a’. Weymouth always uses a capital ‘A’, Douay uses a capital ‘A’ occasionally. The KJV and modern versions do not use a capital ‘A’.
As your question may not relate to a capital ‘A’ or small ‘a’ used by translators, but perhaps to the idea that the ‘A’ relates to the 12 apostles and the ‘a’ relates to others, I add the following:
Apostle (Greek ‘apostolos’) means ‘one sent’. In the King James Version it is translated ‘apostle’ some 78 times. The 3 occasions where it is not translated as ‘apostle’ are:
Jesus chose twelve apostles from among his disciples.
Luke 6:13 And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles
It is clear that the eleven (without Judas) were apostles in a special sense. They were ‘ones sent’ in receiving the commission from Christ.
Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
Acts 1:2 Until the day in which he was taken up, after that he through the Holy Spirit had given commandments unto the apostles whom he had chosen
The number is subsequently made up to twelve with Matthias replacing Judas.
Acts 1:21 Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us, 22 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. 23 And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias. 24 And they prayed, and said, Thou, Lord, which knowest the hearts of all men, shew whether of these two thou hast chosen, 25 That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place. 26 And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.
All had been with Jesus throughout his ministry and were baptised with the Holy Spirit at Pentecost.
John 14:26 But the Comforter, which is the Holy Spirit, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
The converts made by their preaching at Pentecost continued in the apostles’ doctrine and fellowship.
Acts 2:41 Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.42 And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers.
From this we see that the twelve apostles were the acknowledged source of doctrine and the behaviour or conduct that constituted fellowship.
Acts 15:2 When therefore Paul and Barnabas had no small dissension and disputation with them, they determined that Paul and Barnabas, and certain other of them, should go up to Jerusalem unto the apostles and elders about this question.
Here a distinction is made between the apostles and elders.The apostles could confer Holy Spirit gifts by the ‘laying on of hands’.
Acts 8:18 And when Simon saw that through laying on of the apostles’ hands the Holy Spirit was given, he offered them money
Demonstrated in the context is that Philip, although full of the Holy Spirit himself and able to perform miracles, could not confer Holy Spirit gifts by the ‘laying on’ of his hands. The apostles Peter and John had to travel from Jerusalem to lay their hands on them. There is clearly a difference in this respect between the apostles and Philip.
Paul is specially chosen, instructed and sent by Jesus Christ to be the apostle to the gentiles.
1 Corinthians 1:1 Paul, called to be an apostle of Jesus Christ through the will of God
Galatians 1:11 But I certify you, brethren, that the gospel which was preached of me is not after man. 12 For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ.
Acts 22:21 And he said unto me, Depart: for I will send thee far hence unto the Gentiles.
His standing as an apostle is equal to the twelve.
2 Corinthians 12:11 in nothing am I behind the very chiefest apostles, though I be nothing. 12 Truly the signs of an apostle were wrought among you in all patience, in signs, and wonders, and mighty deeds.
Paul could confer Holy Spirit gifts by the ‘laying on’ of his hands
Acts 19:6 And when Paul had laid his hands upon them, the Holy Spirit came on them; and they spake with tongues, and prophesied.
Paul speaks of Jesus’ brother James (James, John’s brother being dead) as an apostle.
Galatians 1:19 But other of the apostles saw I none, save James the Lord’s brother.
We might speculate that his apostleship was conferred by Christ as Paul’s was.
1 Corinthians 15:7 After that, he was seen of James; then of all the apostles.
Titus is not called an apostle. He was chosen by Paul as a travelling companion and charged with the task of ordaining elders in Crete and setting the ecclesias in order.
Titus 1:4 To Titus, mine own son after the common faith: Grace, mercy, and peace, from God the Father and the Lord Jesus Christ our Saviour. 5 For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and ordain elders in every city, as I had appointed thee
Paul describes him thus:
2 Corinthians 8:23 Whether any do enquire of Titus, he is my partner and fellowhelper concerning you
Timothy is not called an apostle. He was chosen by Paul as a travelling companion and circumcised to enable him (we might reasonably suppose) to preach in synagogues with Paul. He was given the oversight of the ecclesia at Ephesus.
Acts 16:1 Then came he to Derbe and Lystra: and, behold, a certain disciple was there, named Timotheus, the son of a certain woman, which was a Jewess, and believed; but his father was a Greek: 2 Which was well reported of by the brethren that were at Lystra and Iconium. 3 Him would Paul have to go forth with him; and took and circumcised him because of the Jews which were in those quarters: for they knew all that his father was a Greek.
1 Timothy 1:3 As I besought thee to abide still at Ephesus, when I went into Macedonia, that thou mightest charge some that they teach no other doctrine,
Paul does not class him as an apostle.
Colossians 1:1 Paul, an apostle of Jesus Christ by the will of God, and Timotheus our brother
Barnabas first appears in Acts 4:36.
Acts 4:36 And Joses, who by the apostles was surnamed Barnabas, (which is, being interpreted, The son of consolation,) a Levite, and of the country of Cyprus, 37 Having land, sold it, and brought the money, and laid it at the apostles’ feet.
Barnabas was specially selected with Paul through the Holy Spirit and sent on missionary work. This may be the reason why he is described as an apostle.
Acts 13:2 As they ministered to the Lord, and fasted, the Holy Spirit said, Separate me Barnabas and Saul for the work whereunto I have called them.
Barnabas is described as an apostle in Acts 14:14.
Acts 14:14 Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out
Judas (who had the gift of prophecy) is not called an apostle but rather one of the companions of the apostles at Jerusalem.
Acts 15:22 Then pleased it the apostles and elders, with the whole church, to send chosen men of their own company to Antioch with Paul and Barnabas; namely, Judas surnamed Barsabas, and Silas, chief men among the brethren:
Acts 15:32 And Judas and Silas, being prophets also themselves, exhorted the brethren with many words, and confirmed them.
We note the distinction Paul makes between apostles and prophets
1 Corinthians 12:28 And God hath set some in the church, first apostles, secondarily prophets...
Andronicus and Junia
Andronicus and Junia are not necessarily apostles themselves but perhaps rather well known to the apostles.
Romans 16:7 Salute Andronicus and Junia, my kinsmen, and my fellowprisoners, who are of note among the apostles, who also were in Christ before me.
There are no apostles today for the signs of an apostle that Paul speaks of are not in evidence. Apostles are no longer necessary because the scriptures are complete and the gospel and doctrine taught by the apostles are written down for our learning.
John 20:31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 17 That the man of God may be perfect, throughly furnished unto all good works.
I hope you have found this helpful.
May God bless you,