Barnabas and Paul are named as apostles in v.14 of Acts 14:12-14.
Yet there are only twelve apostles of the Lamb with Paul replacing Judas.
And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb. (Rev 21:14)
So how do we reconcile this with Barnabas and Matthias being called apostles?
1. Matthias was a temporary fill-in for Judas, as we see from the rest of the NT.
2. For Barnabas this meaning of "apostle" would be fitting: a delegate, messenger, one sent forth with orders.
3. Christ is also called an Apostle (Heb 3:1). However, not in the sense of the twelve, but as the Messenger from God. He is also called"the Messenger of the Covenant".
Behold, I will send my messenger [John the Baptist], and he shall prepare the way before me: and the Lord, whom ye seek, shall suddenly come to his temple, even the Messenger of the Covenant, whom ye delight in: behold, he shall come, saith the LORD of hosts. (Mal 3:1)
Here "the Lord" (Jesus Christ) is called the Messenger of the Covenant, which could also be stated as "the Apostle of the Covenant". However, the reference is to the Second Coming of Christ (and in this verse both the first and the second comings are intimated).