Why did the disciples need exactly twelve apostles?
Twelve apostles were required because Jesus promised that they would sit on twelve thrones, ruling the twelve tribes of Israel in His earthly kingdom. Since Judas had abdicated his place, leaving only eleven, a twelfth man was needed; Matthias was chosen to fill the vacant slot.
Les explains that when the Son of Man sits on the throne of His glory, which Scripture places on the earth and centered in Jerusalem, the eleven are told they also will sit upon twelve thrones, ruling the twelve tribes of Israel. Peter was uptight because Judas had abdicated his spot, and if the King was coming at any moment, how could twelve thrones be filled with only eleven? So the first order of business was to get a replacement for Judas. They chose Matthias, because if they were going to rule twelve tribes, they had to have twelve men; Matthias filled the slot left open by Judas. All of this shows God promising Israel this earthly kingdom.