How many Disciples Saw Jesus After his Resurrection?
How could Jesus appear unto the "twelve" if Judas was dead and Mathias was not yet chosen as a replacement?
The number that Paul gives to us in 1 Corinthians is accurate. The Lord did appear unto the twelve disciples. However, Paul is not referring to the original twelve disciples, but rather to the original eleven disciples plus Matthias. How do I know that the Lord appeared unto Matthias after his resurrection? Simple. This was listed as a requirement for the disciple that would replace Judas listed in Acts Chapter 1:
20 For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his bishoprick let another take.
21 Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us,
22 Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection.
23 And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias.
Notice how Mathias has to be a witness with us of his resurrection. So, Mark is correct in saying that Jesus appeared unto "the eleven" since that is how many of them were disciples at the time of the appearing. But Paul is also correct when he writes later to the Corinthians and says that he appeared unto "the twelve" since that is how many of the current twelve disciples he had appeared unto. There is no contradiction in the scriptures.
Keep in mind that it is perfectly acceptable to refer to prior events using current titles. For example, we often refer to the "President's childhood" or where the "President went to college." Obviously, the President was not actually the President as a child or while attending college, we are simply applying a current title to a previous event.