In a previous post I argued against the notion, advocated by a fringe New Testaments scholar and assorted internet skeptics, that 1 Corinthians 15:3-11 is an interpolation. Therein, one argument I made was that although a later Christian interpolation would have post-dated the gospel accounts of the resurrection appearances, 15:3-11 appears to be independent of them. One example I mentioned was that 15:5 refers to Jesus' appearance to "the twelve," whereas the gospels clearly record that Judas was dead at the time of Jesus' appearances. More to the point is that the synoptic Gospels all specifically refer to Jesus appearing to "the eleven" despite the fact that they earlier refer to the inner circle of disciples as "the twelve." (Matt. 28:16 Mark 16:4; Luke 24:33). Acts also has "the eleven" deciding how to pick a new member to bring their number back up to twelve. (Acts 1:26). Accordingly, it is unlikely that the author of 15:5 was written by someone who was familiar with the synoptic Gospels and Acts, and to a lesser extent the Gospel of John.
In response to this argument, I have been accused of advocating a "conflict" between Paul and the Gospels. Was Paul ignorant of Judas' betrayal? Was Judas himself simply a later fabrication invented by the gospel authors? I reject such conclusions on other grounds, including Paul's mention of betrayal in 1 Corinthians 12. But for now it is enough to demonstrate that Paul's reference to "the twelve" cannot be used as evidence that he is ignorant of Judas' betrayal and death.
My typical response to this charge of disagreement has been to offer two possible explanations for the discrepancy. First, it is possible that Paul is being anachronistic. He knows Judas betrayed Jesus and had killed himself by the time of the resurrection appearances, but he also knew that Matthias had been added to "the twelve" and had been present for Jesus' appearances. After all, to be a member of "the twelve" Matthias had to have participated in Jesus' ministry up to His ascension. (Acts 1:22). Thus, when Paul says "the twelve" he is counting Matthias among them. Second, "the twelve" is obviously a group of symbolic importance (the Twelve Tribes of Israel) and may not have had a fixed membership. If one disciple was sent out or left the group, a number was added to it so as to keep the group's symbolic significance. This finds support in the fact that "the eleven" were quick to bring their number up to "the twelve" shortly after Jesus' ascension. Thus, when Paul refers to "the twelve" he is referring to the group irrespective of how many members were in it at any specific point in time.
It is possible that Paul was prompted by both considerations. In any event, I ran across some evidence adding support to the second explanation in a fine commentary on 1 and 2 Corinthians by Craig S. Keener. Therein he notes:
The "twelve" was a title for Jesus's closest followers, which Jesus as leader of a renewal movement undoubtedly chose to evoke the biblical tribes of Israel (cf. 1QS 8.1-2). Despite the number, one would hardly expect the Gospel writers to have invented apostasy by one of the twelve (betrayal from an inner circle constituted an embarrassment in ancient society); numerical group titles were common, and often remained even when numbers fluctuated.
Keener, 1-2 Corinthians, page 124.
Keener provides a number of examples in a footnote, including Athen's "eleven" (Xenephon Hell. 1.7.10; 2.3.54); "thirty" (Xenephon Mem. 4.4.3); and "5000" really controlled by "400" (Plutarch Alcib. 26.2; 27.1); or Rome's "ten" (Suetonius Aug. 36) and "fifteen" (Julius 79.2); and especially the "one hundred," with fluctuating numbers (Cicero Agr. 2.17.44; Statius Silvae 1.4.24). Keener, op. cit., page 124 n. 272.
None of this proves, of course, that Paul knew that there was fluctuation in the numbers of "the twelve." But it does indicate that we cannot infer from the mention of "the twelve" that Paul was ignorant of the fluction of the numbers in that group.