"And the wall of the city had twelve foundations,

and on them were the twelve names

of the twelve apostles of the Lamb." (Revelation 21:14)

It is no coincidence that there are twelve apostles. Jesus clearly intended it to be so! The 12 apostles are to be seen as the progenitors of The church, Paul tells us,  "the household of God, built on the foundation of the apostles and prophets" (Ephesians 2:19-20).

We see the number 12 throughout Revelation relating to both the apostles and the tribes of Israel:

So who are the twelve apostles of the Lamb? To whom does the "twelve apostles of the Lamb" refer? Is this the original twelve apostles? or does Mathias replace Judas? Or does this refer to someone or something else?

Paul and Barnabas are both called apostles in the book of Acts:

14 But when the apostles Barnabas and Paul heard this, they tore their clothes and ran in among the multitude, crying out  -Acts 14:14 (NKJV)

The Holy Spirit had set them apart earlier in Antioch:

1 Now in the church that was at Antioch there were certain prophets and teachers: Barnabas, Simeon who was called Niger, Lucius of Cyrene, Manaen who had been brought up with Herod the tetrarch, and Saul. 2 As they ministered to the Lord and fasted, the Holy Spirit said, “Now separate to Me Barnabas and Saul for the work to which I have called them.” 3 Then, having fasted and prayed, and laid hands on them, they sent them away. -Acts 13:1-3 (NKJV)

So neither Paul nor Barnabas were self-proclaimed apostles but were set apart by the Holy Spirit. Note that even though Acts refers to them as apostles, there is still a distinction made between them and the other twelve apostles, with one example being in Acts 15 where Paul and Barnabas go back to Jerusalem to consult with the apostles and elders about the matter of circumcision: