Read Acts 1:15-17 and 21-26
15 A few days later there was a meeting of the believers, about 120 in all, and Peter stood up to speak. 16“My fellow-believers,” he said, “the scripture had to come true in which the Holy Spirit, speaking through David, made a prediction about Judas, who was the guide for those who arrested Jesus. 17Judas was a member of our group, for he had been chosen to have a part in our work.”
21–22 “So then, someone must join us as a witness to the resurrection of the Lord Jesus. He must be one of the men who were in our group during the whole time that the Lord Jesus travelled about with us, beginning from the time John preached his message of baptism.”b until the day Jesus was taken up from us to heaven
23 So they proposed two men: Joseph, who was called Barsabbas (also known as Justus), and Matthias. 24Then they prayed, “Lord, you know the thoughts of everyone, so show us which of these two you have chosen 25to serve as an apostle in the place of Judas, who left to go to the place where he belongs.” 26Then they drew lots to choose between the two men, and the one chosen was Matthias, who was added to the group of eleven apostles.
The disciples need to choose someone to make up the twelve after the loss of Judas. They are famous for drawing lots on this decision - not however that the requirements were that the 12th apostle be someone who had journeyed with Jesus from his baptism to his resurrection.
The symbolic significance of 12 apostles is that they were to represent the 12 tribes of Israel.
God used the 12 to oversee the spread of the good news about Jesus throughout the world. Today God helps us choose leaders, ministers, bishops, pastors, superintendents, preachers to continue that work.
Pray for the people that God chooses and uses.