Aside from the original 12 Apostles, an Apostle is also a Ministry gift of the Church as a missionary or other special responsibilities.
General references to Apostles:
Now there were in the church that was at Antioch certain prophets and teachers; as Barnabas, and Simeon that was called Niger, and Lucius of Cyrene, and Manaen, which had been brought up with Herod the tetrarch, and Saul.
2 As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them.
3 And when they had fasted and prayed, and laid their hands on them, they sent them away. (Act 13:1-3 KJV)
28 And God hath set some in the church, first apostles, secondarily prophets, thirdly teachers, after that miracles, then gifts of healings, helps, governments, diversities of tongues.
29 Are all apostles? are all prophets? are all teachers? are all workers of miracles? (1Co 12:28-29 KJV)
11 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
12 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ: (Eph 4:11-12 KJV)
Specific examples:
Barnabas
14 Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out, (Act 14:14 KJV)
Andronicus and Junias
7 Salute Andronicus and Junia, my kinsmen, and my fellowprisoners, who are of note among the apostles, who also were in Christ before me. (Rom 16:7 KJV)
Titus and Brethren
23 Whether any do enquire of Titus, he is my partner and fellowhelper concerning you: or our brethren be enquired of, they are the messengers of the churches, and the glory of Christ. (2Co 8:23 KJV)
James, brother of Jesus
19 But other of the apostles saw I none, save James the Lord’s brother. (Gal 1:19 KJV)
Epaphroditus
25 Yet I supposed it necessary to send to you Epaphroditus, my brother, and companion in labour, and fellowsoldier, but your messenger, and he that ministered to my wants. (Phi 2:25 KJV)