Albert Barnes on Mark 3:14

He ordained twelve. The word rendered ordained, here, does not express our notion of ordination to the ministry. It means, literally, "he made," or he appointed twelve to be with him.

Twelve. The reason why twelve were chosen was, probably, that such a number would be deemed competent witnesses of what they saw; that so many could not be supposed to be imposed upon; that they could not be easily charged with being excited by sympathy, or being deluded, as a multitude might; and that, being destined to go into all the world, a considerable number seemed indispensable. Perhaps, also, there was some reference to the fact that twelve was the number of the tribes of Israel.