1 Corinthians 15:
3 For what I received I passed on to you as of first importance[a]: that Christ died for our sins according to the Scriptures, 4that he was buried, that he was raised on the third day according to the Scriptures, 5and that he appeared to Peter, and then to the Twelve.

Suggest it is not clear.  In Acts the apostles elect another guy to replace Judas.  He would have been in the 12 that Paul was familiar with.

That’s right John.  We had a sermon on this passage this morning.  Matthias became one of the 12 and he saw all that the disciples had seen, which was one of the critieria for choosing him.

Acts 1:21-22
21Therefore it is necessary to choose one of the men who have been with us the whole time the Lord Jesus went in and out among us, 22beginning from John’s baptism to the time when Jesus was taken up from us. For one of these must become a witness with us of his resurrection.”

Ray,
It’s not that complicated really. I’d suggest something like this: Paul writes 1 Corinthians around AD 55. At this stage - some 20years after Christ’s resurrection - “the twelve” would have been firmly understood not to include Judas Iscariot, and to include Matthias. Now Matthias, as Craig and John have pointed out, officially became part of “the twelve” after Christ’s ascension, but had been a witness to all that had happened with Jesus, including Jesus’ resurrection. So there’s no real problem with Paul’s statement.