NOTE: "disciple" means "learner". "apostle" means "one sent". Jesus chooses a special group of His learners who are called "the Twelve" and designates them with power and authority which was recognized in the early church. Matthias is selected to take Judas' place (Acts 1). Paul is added as "one born out of due season" (1 Corinthians 15).
BARNES: In selecting “twelve” at first, it is probable that the Saviour was somewhat guided by the number of the tribes of Israel. Twelve was, with them, a well-known number, and it was natural that he should select one for every tribe. Their office was clearly made known. They were to heal the sick, cast out devils, raise the dead, preach the gospel. They were to be with him to receive his instructions, to learn the nature of his religion, be witnesses to his resurrection, and then to bear his gospel around the globe. The number twelve was the best number for these purposes that could be selected. It was sufficiently “large” to answer the purpose of testimony, and it was “so small” as not to tend to disorder, or that they could easily be divided into parties or factions. They were not learned men, and could not be supposed to spread their religion by art or talents. They were not men of wealth, and could not bribe men to follow them. They were not men of rank and office, and could not compel people to believe. They were just such men as are always found the best witnesses in courts of justice - plain men, of good sense, of fair character, of great honesty, and with favorable opportunities of ascertaining the facts to which they bore witness. Such men everybody believes, and especially when they are willing to lay down their lives to prove their sincerity.
It was important that the Saviour should choose them early in his ministry, in order that they might be fully acquainted with him; might treasure up his instructions, and observe his manner of life and his person, so that, by having been long acquainted with him, they might be able to testify to his identity and be competent witnesses of his resurrection. No witnesses were ever so well qualified to give testimony as they, and none ever gave so much evidence of their sincerity as they did. See Act_1:21-22.