We see at 1 Cor 15:3-5:
For I handed on to you as of first importance what I in turn had received: that Christ died for our sins in accordance with the scriptures, and that he was buried, and that he was raised on the third day in accordance with the scriptures, and that he appeared to Cephas, then to the twelve.
But, we see Matthias being elected as replacement for Judas Iscariot only after the ascension of Jesus (See Acts 1:9 [Jesus' ascension] & Acts 1:26 [Matthias chosen]).
My question is: whom does Paul refer to as "the twelve" when there were only eleven apostles left in the group before the ascension of the Lord? What is the explanation offered by Catholic Church for the said statement of Paul?