“28 Jesus said to them, Truly, I say to you, in the new world, when the Son of Man will sit on his glorious throne, you who have followed me will also sit on twelve thrones, judging the twelve tribes of Israel.” - Matt 19
Paul's mission was separate from the 12. As is apparent when Romans through Philemon is read. He constantly refers to "my gospel", "in me FIRST", the "gospel of the grace of God that was committed to ME", etc.
Choosing Matthias was all about the day of Pentecost and what Peter and the 10, soon to be 11 with Matthias, was expecting on that day. Hint: It was all about Israel and the eminent return of Christ to set up His Kingdom, and the twelve thrones being complete in order to judge the twelve tribes of Israel.