Twelve is the number of God's covenant people. Jacob had twelve sons, who became the twelve tribes of Israel — the whole nation, organised and complete. The number recurs through Israel's worship: twelve stones on the high priest's breastplate, twelve loaves of the bread of the Presence, twelve stones taken from the Jordan.
Jesus deliberately chose twelve apostles, signalling that he was forming the true, renewed Israel around himself. And Revelation's new Jerusalem is stamped with twelve: twelve gates bearing the names of the twelve tribes, twelve foundations bearing the names of the twelve apostles. Twelve gathers up the whole people of God, old covenant and new, into one complete and governed community — the family God has been building from the start.
The Number Twelve in Scripture
And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles;