Who were “the twelve” that Jesus appeared to, as mentioned in 1 Corinthians 15:5?

The appearance being mentioned in 1 Corinthians 15:5 seems to be the one recorded in John 20:26-29, which involved Thomas. However, this refers to the apostles as a group and likely included Matthias.
 

In discussing the resurrection, Paul wrote of Jesus’ appearances to humans after He was raised. The apostle said that Christ “appeared to Cephas, then to the twelve. After that he appeared to upward of five hundred brothers.”—1 Corinthians 15:5, 6.
 

From among those who followed him as disciples, Jesus selected 12 apostles. (Matthew 10:2-5) Judas Iscariot was one of the 12, but he turned traitor, betrayed Jesus, and then hanged himself. (Matthew 26:20-25; 27:3-10) So at the time of Christ’s death and resurrection, there were only 11 faithful apostles of the original 12. Jesus appeared to various disciples between his resurrection and ascension to heaven. 

Thereafter the apostles recognized the need to replace Judas. With divine guidance, Matthias was selected, and “he was reckoned along with the eleven apostles.”—Acts 1:6-26.
 

Some have thus wondered why Paul would write that Jesus appeared to “the twelve,” since at that time Judas was dead and Matthias had not yet been selected. Being specific, there were then only “eleven apostles” that had originally been designated and sent forth by Jesus.—Luke 6:13-16.
 

It is normal to speak of a group collectively even if one member is absent. (“The board of directors decided . . . ” “The body of elders met . . . ”) So the term “the twelve” may well have been used in reference to the whole group of apostles, even if one or two were absent on an occasion. (Compare Acts 6:1-6.) When Jesus first appeared to the disciples in a locked room, “Thomas, one of the twelve, . . . was not with them.” Eight days later he was present and had any uncertainty settled. (John 20:19-29) Though Matthias had not then been designated to replace Judas, he was a longtime disciple. (Acts 1:21, 22) Since he was closely associated with the original apostles and shortly thereafter was “reckoned along with” them, the retrospective comment about Jesus’ appearance to “the twelve” likely included Matthias.