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)
There were 12 original Apostles that Jesus commissioned to establish the Church and spread the gospel.
General References of the 12 Apostles.
33 And with great power gave the apostles witness of the resurrection of the Lord Jesus: and great grace was upon them all.
34 Neither was there any among them that lacked: for as many as were possessors of lands or houses sold them, and brought the prices of the things that were sold,
35 And laid them down at the apostles’ feet: and distribution was made unto every man according as he had need.
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. (Act 4:33-37 KJV)