What did Jesus promise the twelve disciples would do in His kingdom?

Jesus promised the twelve who followed Him that in the regeneration, when the Son of Man sits on the throne of His glory and sets up His kingdom, they also would sit on twelve thrones, judging or ruling the twelve tribes of Israel. Judas was not included in this promise.

Les explains that Jesus was speaking directly to Peter and the other disciples who had followed Him; Judas was not included. He says that “regeneration” means the time when things are put back as they were originally. When the Son of Man sits on the throne of His glory—that is, when He has set up His Kingdom and is now the King—the twelve will also sit upon twelve thrones, judging or ruling the twelve tribes of Israel. Les notes this is plain language: Jesus assures these men that they have eternal life and belong to Him, while Judas is excluded.

Matthew 19:28

And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.

What was the reward Jesus promised the twelve apostles for serving Him?

Jesus promised the Twelve that when the Son of Man sits on the throne of His glory, they also would sit upon twelve thrones, judging the twelve tribes of Israel. This was their reward for serving Him diligently during the three years of His earthly ministry, and it had nothing to do with their salvation.