It all depends on the definition of the word.
The New Testament clearly describes how Jesus appointed 12 “apostles” in his lifetime to preach the good news to all creation.
Yet, the Catholic Church often uses the word “apostle” in reference to other saints as well, raising the question, “How many apostles are there?”
The English word “apostle” is derived from the Greek apostello, “to send forth.” An apostle is one who is “sent” by God to preach the Gospel to the nations. This is typically used in reference to the original 12 apostles who were sent into the world by Jesus.
However, the New Testament names several other disciples of Christ as “apostles.”
First of all, after Jesus’ ascension into heaven the remaining eleven (minus Judas, the betrayer) appoint Matthias as a successor. They explain how they needed, “one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us, beginning from the baptism of John until the day when he was taken up from us—one of these men must become with us a witness to his resurrection” (Acts 1:21-22). Matthias had encountered Christ and was now given the same task as the eleven.
Progressing through the New Testament there appears to be more apostles, such as St. Paul, who applies this title to himself, “Am I not free? Am I not an apostle? Have I not seen Jesus our Lord? Are not you my workmanship in the Lord?” (1 Corinthians 9:1)
Besides Matthias and Paul, another individual is named an apostle, “when the apostles Barnabas and Paul heard of it …” (Acts 14:14). Little is known about Barnabas, but the fact that the author of Acts labels him an apostle suggests that Barnabas had similar qualifications.
Officially the Church stops there and labels the following saints as “apostles” according to this definition: Peter, Andrew, James, son of Zebedee, John, Philip, Bartholomew, Thomas, Matthew, James, son of Alphaeus, Judas Thaddeus (Jude), Simon the Zealot, Matthias, Paul and Barnabas.
At the same time the Church will sometimes labels other saints as “apostles” who do not fit the biblical description. Instead, these apostles are in accord with the basic definition of the word, a person who is “sent” to preach the Gospel. The “12 Apostles of Ireland” is one such example, as well as the “Apostle to the Apostles,” Mary Magdalene.
In the end, the title is an important one that has various meanings, but all come back to the essential mission of an apostle as an evangelizer who brings the good news of Jesus Christ to all they encounter.