Many people consider the 12 apostles as a class apart into which they substitute Paul for Judas. That is backed up by the fact that John says that the wall of the “the holy city, Jerusalem” which descended from heaven had twelve foundations on which were the names of the twelve apostles (Rev. 12:14). However the term apostle is used in a more general way as well. For instance in Romans16:7 two people are mentioned (one a woman) who are called apostles but don’t belong to the twelve.
The word apostle is a direct transliteration of the word “apostolos” in Greek meaning “sent out one”. In Latin the word is “misseo” from which we get the word “missionary”. Our common use of the word “missionary” to describe someone sent out from the church to plant (or assist in planting) churches elsewhere has no parallel in Scripture other than in the use of the word “apostle” since it means the same thing and would have been read in that way by the original readers of, for example, Luke 10:1:
10 After these things the Lord appointed seventy others also, and sent them two by two before His face into every city and place where He Himself was about to go.
So when I say I believe in apostolic ministry today I am saying that I believe that every missionary sent by God to plant churches is partaking in an apostolic ministry today. Some missionaries are not sent by God and others are assisting in church growth by bringing another gifting to the place they are sent to such as prophecy (often manifested in preaching), teaching, etc. (see 1 Cor. 12: 28-29, Ephesians 4:11-12). So the word “missionary” as we commonly use it does not just include the apostolic ministry but these other ministries as well.
There are therefore much fewer apostles (in the biblical ministry sense) who are missionaries than there are missionaries in general. However I believe that this ministry does exist and is happening today all over the world.
Of course people are free to disagree with me on this. I believe the Scripture in our age of grace is not prescriptive on such
matters. The Holy Spirit doesn’t mind much what you call yourself as long as you are working with Him and not against Him in His purposes. Better to be working in the field planting churches or assisting them than sitting around discussing what you call someone who is!