Why (and when) did the Apostles stop requiring exactly twelve apostles? Why was James not replaced?

God has chosen or appointed the position of the ones HE wants to fulfill HIS plan. Judas, for whatever reason chose to step out of his position. Now it has to be filled to carry on THE PLAN. When James died, he never left the plan. What is the plan? That you might have eternal life. When James died, he took that position God gave him with him. It does not have to be filled. God gives everyone his own personal position in HIS plan. We are obligated to seek and find that position GOD has for us and stay there. HE has chosen you to in HIS plan. Choose ye this day and stay in it.

Matt. 19:27,28 & Lk 22 speak of the Apostles ruling the 12 tribes of Israel

27 “Look,” Peter replied, “we have left everything to follow You. What then will there be for us?” 28 Jesus said to them, “Truly I tell you, in the renewal of all things, when the Son of Man sits on His glorious throne, you who have followed Me will also sit on twelve thrones, judging the twelve tribes of Israel."

Also, Luke 22:28-30

"28 But you are those who have continued with Me in My trials. 29 And I bestow upon you a kingdom, just as My Father bestowed one upon Me, 30 that you may eat and drink at My table in My kingdom, and sit on thrones judging the twelve tribes of Israel."

From the commentary found @ BibleStudyTools.com

Not only will the saints reign with Christ, they will also judge with Him (Rev. Rev. 20:4+). It is important to realize that even though the saints will perform judgment, this will not be the dominant activity during the Millennial Kingdom, at least not in the sense of meting out punishments. [See George H. N. Peters, The Theocratic Kingdom (Grand Rapids, MI: Kregel Publications, 1978, 1884), 2:352-361]

It appears that Jewish saints—the resurrected apostles—will judge over the twelve tribes, the nation of Israel. At that time, all ten tribes will be present and accounted for.

Acts 1:12-26 details the appointing of the Apostle to replace Judas.

Especially look at verse 16, then 20... quoted here:

20 For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his bishoprick let another take.

Continuing, verses 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.

So then, from this account in Acts it seems quite clear that the reason they were deliberating over two, though it could easily be understood that there were more who had been with them from the beginning, was because of their interpretation of Psalms as it related to Judas. They felt, obviously through the guiding of the Holy Spirit as they had just received it as Jesus had said they would, that this was the correct interpretation and course of action.

As far as when:

Some would say the events of the early chapters of Acts occurred during the decade of the AD 30's. The Book of Acts (of the Apostles) was supposedly written between 61-64 A.D. according to this site https://gotquestions.org/Book-of-Acts.html

The controversy that might be had—could be had at Shaul/Paul's assertion that He was an apostle, equal in every way to those hand picked by Jesus. Romans 1:1, 2 Timothy 1:1, Ephesians 1:1, 1 Corinthians 1:1, all include his greeting saying he is an Apostle specifically of Jesus, the Christ.

Shaul's first missionary journey, according to one source is dated between 46-48 A.D. According to the same timeline, the author has Herod Agrippa I's order for James' execution occurring at the year 44 A.D.

So if you wanted to make a case for Paul replacing James the onus would be yours, but the Scriptures do not seem to hint that a tradition of having precisely 12 living Apostles at any one time after the appointment of Matthias, existed.

They did replace him!

Galatians 1:19 says: "But other of the apostles saw I none, save James the Lord’s brother." [KJVPCE]

Although there is no explicit reference, we can clearly see that James the Lord's brother is called an apostle. Putting two and two together we see that they must have chosen him to replace James the brother of John who was killed, just like they did with Judas and Matthias.

Galatians was written by Paul after James was killed, so it is understood by this stage that James the Lord's brother is an apostle.

By the way, for those who don't know, there are 3 James:

• James the son of Zebedee (who is killed)
• James the son of Alphaeus
• James the Lord's brother (Yes, Marry had other children after Jesus was born as the bible clearly teaches, she was not a perpetual virgin as Catholic tradition says)

James the Lord's brother is probably also the one who wrote The General Epistle of James.

Although he was an unbeliever at first, and did not believe in Jesus while he was alive along with Jesus' other brethren [John 7:5], we clearly see in Acts 1:14 that Jesus' brethren had converted after his resurrection, as we see them praying with one accord along with Marry and the disciples.

The Twelve Apostles elected by Jesus were not meant to live forever and rule the earth. They represented the fathers of Christianity and the church.

Judas was a betrayer. He betrayed his master Jesus, which is the same as betraying the whole group of twelve, and it implies that he is no longer a part of the group of the Twelve Apostles. Judas deserted the company of Twelve. It was now necessary to replace the office of Judas, possibly because they believed that 12 was the divinely appointed number by Jesus Christ and it should be maintained. The number twelve might also represent the twelve tribes of Israel. Thus, it seems like it was necessary to continue the tradition of the number Twelve.

The death of James did not end his position in the Twelve as he was not a betrayer like Judas. In fact, all the apostles (according to tradition) were martyred. They are now all dead and probably are now in Heaven, occupying the Twelve Thrones reserved for them in Heaven (my speculation).

And, Jesus did not elect Twelve Popes, rather Twelve Apostles.