Luke 6:17,20: "And he came down with them, and stood in the plain, and the company of his disciples, and a great multitude of people...came to hear him.. And he lifted up his eyes on his disciples and said..."
Matthew 27:3-5 tells of the death of Judas, which means 11 were left. Matthew 28:16 spells out that there were 11 men. And Acts 1:9-26 tells of the replacement. So what does 1Corinthians 15:5 mean when Paul says “the Twelve”? It is not contradictory, rather Paul is using a title of the group: even if some are missing they will always be known as “the Twelve”.