The Apostles were the first bishops. They ordained many more bishops to carry on the Church’s work during the course of time. It would be hard to administer a worldwide Church with only 12 bishops.
It is true that on the day of Pentecost, there were only eleven Apostles (bishops). But, also, there were only 3,000 Christians (Acts 2:41). As the Church grew, so did Her leadership.
If someone wishes to claim that the number of bishops should be fixed, s/he would be inconsistent if s/he did not also claim the number of Christians should also be fixed. If the number of Christians may grow, why should not the number of Bishops also grow?
Acts chapter tells us the apostles elected Matthias to replace Judas.
Jesus also gave many parables to describe how the Church would grow, such as the mustard seed, the leaven in a measure of wheat etc., and prepared the Apostles to spread their ministry by ordaining others.
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