Did Jesus appear post-resurrection to twelve apostles? 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] &amp; 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?
While Matthias was not an apostle at the time that Jesus appeared to the other eleven apostles, the Scriptures do indicate that Matthias was a disciple during Jesus' ministry.

According to Peter’s words (Ac 1:21, 22), Matthias had been a follower of Christ throughout Jesus’ three-and-a-half-year ministry, had been closely associated with the apostles, and was quite likely one of the 70 disciples or evangelists whom Jesus sent out to preach. (Lu 10:1) After his selection, he was “reckoned along with the eleven apostles” by the congregation (Ac 1:26), and when the book of Acts immediately thereafter speaks of “the apostles” or “the twelve,” Matthias was included.​—Ac 2:37, 43; 4:33, 36; 5:12, 29; 6:2, 6; 8:1, 14; see PAUL. [article &quot;Matthias&quot; printed in &quot;Insight on the Scriptures&quot; by Watch Tower Bible and Tract Society of Pennsylvania]

So, Paul was recognizing Matthias as an apostle even though his appointment to that position had not be made yet.
(While not an answer from the Catholic perspective, this does give an answer from the Bible itself.)