My guess is that 12 of them are Apostles.
In 1 Peter 5:1, Peter refers to himself as an elder.
In Matthew 19:28, it says:
Jesus said to them [Peter and his other disciples], "Truly, I say to you, in the new world, when the Son of man shall sit on his glorious throne, you who have followed me will also sit on twelve thrones, judging the twelve tribes of Israel.
I don’t know about the other 12. Maybe they are prophets since prophets are often mentioned along with the apostles:
Ephesians 2:19-20 says:
So then you are no longer strangers and sojourners, but you are fellow citizens with the saints and members of the household of God, built upon the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone"
Revelation 18:20 says:
Rejoice over her, O heaven, O saints and apostles and prophets, for God has given judgment for you against her!
I guess we will have to wait until we get there to find out for sure.