Bede — on Rev 7:4 (Commentary on Revelation)

“And I heard the number of those who were sealed, etc. This finite number signifies the innumerable multitude of the whole Church, which is generated from the patriarchs, either by descent of flesh or by imitation of faith. For if you are Christ's, then you are Abraham's seed (Gal. 3). For the increase of perfection, it pertains that twelve is multiplied twelve times, and to the total is perfected as a thousand, which is a solid squared number, signifying the stable life of the Church. Therefore, it is often figured by the number twelve because it stands firm in the faith of the Holy Trinity in a fourfold world. For thrice four is the perfect number, a double portion. Finally, the apostles, who were chosen to preach the same faith to the world, were twelve, indicating by their number the mystery of their work.”