There is an interesting theological purpose in the historical account in Acts 1:12-26. At first glance it doesn't seem to be anything more than an historical recollection of the apostasy, suicide and replacement of Judas Iscariot, the one who betrayed our Lord. But the place of this record in the book of Acts, the nature of the apostles' prayer, and the Scripture that Peter appeals to all bring a much more significant purpose into view. Jesus has told the apostles to wait for the promised Holy Spirit, whom He would pour out on them ten days later at Pentecost. The Spirit would give them power to fulfill the evangelist ministry in the world. They had been specifically chosen to establish the New Covenant church, the new Israel, throughout the world. As such there would be 12 of them. With His initial choice, Jesus had picked the 11 found in the upper room after his resurrection, and He had chosen Judas. It was obviously no surprise to Him that Judas would betray Him. Peter, in this place, appeals to verses out of two Psalms in order to explain the necessity of replacing Judas:

And in those days Peter stood up in the midst of the disciples (altogether the number of names was about a hundred and twenty), and said, ¹⁶ “Men and brethren, this Scripture had to be fulfilled, which the Holy Spirit spoke before by the mouth of David concerning Judas, who became a guide to those who arrested Jesus; ¹⁷ for he was numbered with us and obtained a part in this ministry.” (Now this man purchased a field with the wages of iniquity; and falling headlong, he burst open in the middle and all his entrails gushed out. ¹⁹ And it became known to all those dwelling in Jerusalem; so that field is called in their own language, Akel Dama, that is, Field of Blood.) “For it is written in the Book of Psalms: ‘ Let his dwelling place be desolate, And let no one live in it’; and, ‘ Let another take his office.

It is extremely important to note that the Holy Spirit predicted the replacement of Judas as an apostle. It was not simply the betrayal that was prophesied of, but the Spirit (incidentally, the same Spirit for whom the apostles were waiting) foretold the apostolic replacement. The question we ought to be asking is,  "Could the 11 apostles not have fulfilled the Great Commission?" Apparently not! Why then did he need to be replaced? There must be 12 apostles because Jesus had reconstituted Israel and was going to establish the New Israel through the 12 new heads of His church. The Old Covenant church was marked by 12 tribes, the New by 12 apostles. This is no incidental detail. Our Lord purposefully planned the work of redemption so that we would be convinced of the Divine origin of the Gospel. The choosing of a twelfth apostle, to replace Judas, was part of this divine plan.