The word "apostle" is recorded 19 times in 19 KJV verses, with its plural appearing an additional 60 times in 59 verses. It is derived from the Greek apostolos (Strong's Concordance #G652) which means someone who is a delegate or messenger, one who is sent forth, or someone who is an ambassador of the Gospel. The only place it is used twice is in 2Corinthians where Paul gives a stern warning regarding those seeking to lead Christians astray.
For such are false apostles, deceitful workers, transforming themselves into the apostles of Christ (2Corinthians 11:13, KJV throughout).
No Limit
The Bible does not limit the designation of apostle, a person specially sent to preach the gospel, only to the twelve men called and chosen by Jesus (Matthew 10:1 - 4, Mark 3:13 - 18). Paul's warning in 2Corinthians 11:13 reveals others were given this holy calling and that they were mimicked by those seeking to take advantage of Christians.
The original 12 disciples were, of course, unique as they were eyewitnesses to Jesus' earthly ministry (John 15:26 - 27). They heard his preaching to the people, saw his miracles, and so on. Although this naturally meant they had a certain authority in the teaching, they shared in the same responsibilities and spiritual gifts given to others with the same calling (e.g. Paul).
Others
A man named Matthias was chosen by the eleven remaining disciples to take the place of Judas after his suicide (Acts 1:26). Paul and Barnabas are named apostles in the Bible, as they were specially selected and sent by God to evangelize. Paul, in fact, had to vigorously defend his right to be considered an apostle based on his calling and the fruits it produced (see 1Corinthians 9, Galatians 2).
As they ministered to the Lord, and fasted, the Holy Ghost (Spirit) said, Separate me Barnabas and Saul for the work whereunto I have called them. And when they had fasted and prayed, and laid their hands on them, they sent them away. (Acts 13:2 - 3).
Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out . . . (Acts 14:14).
Biblical evidence additionally suggests Apollos (1Corinthian 4:6, 9), Silvanus and Timothy (1Thessalonians 1:1, 2:6), as well as Andronicus (a possible distant relative of Paul's) and a person named Junia were also apostles (Romans 16:7). Paul referred to James, the Lord's physical half-brother, as also possessing this calling.
But other of the apostles saw I none, save James the Lord's brother. (Galatians 1:19).
Did You Know . . .
Apostolos is translated "he that is sent" in John 13:16 where Jesus stated, ". . . neither he that is sent greater than he that sent him" (KJV). This is an indirect reference to himself as an Apostle, meaning someone sent by God the Father, which is confirmed in Hebrews 3:1.
Only eight New Testament books out of 27, or about 30%, were written by one of the original 12 apostles. These writers, Matthew, John and Peter, together comprised only 27% of the eleven disciples personally charged by Christ, at his ascension, with the Great Commission (Matthew 28:16 - 20).