Acts 1:22-26
22 Beginning from the baptism of John, unto that same day that He was taken up from us, must one be ordained to be a witness with us of His resurrection.
23 And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias.
24 And they prayed, and said, Thou, Lord, Which knowest the hearts of all men, shew whether of these two Thou hast chosen,
25 That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place.
26 And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.
(KJV)
Matthias was the 12th Apostle that replaced Judas Iscariot.
Apostle Paul was the 13th, which is pretty revealing for those who understand there's technically 13 tribes of Israel and not just 12.