After the crucifixion, and Judas suicide, the Apostles chose Mattias, to replace him so as to keep their number at 12.
But I have always thought of Paul as being the true next Apostle, since Jesus chose him as well....................
They believed God had made that choice
"Then they cast lots, and the lot fell to Matthias;
so he was added to the eleven apostles."
Acts 1:26