Why did the apostles need twelve men instead of eleven?
The eleven were promised they would sit on twelve thrones, ruling the twelve tribes of Israel. When Judas abdicated his place, Peter saw that only eleven thrones would be filled, so the vacant slot had to be taken. They therefore chose Matthias as the twelfth man to complete the promised number.
Les explains that the Son of Man will sit on the throne of His glory, and this earthly kingdom will be centered in Jerusalem, with the nations flowing into it. Because the eleven were told they would sit on twelve thrones, ruling the twelve tribes of Israel, Peter was concerned when Judas's place was empty. He thought the King was coming at any moment, and twelve thrones could not be filled with only eleven men. So Peter's first order of business was to get a replacement for Judas, and they chose Matthias to fill the slot Judas left open. All of this shows God was promising the nation of Israel this earthly kingdom.