No one is willing to make the 12,000 a literal figure as if each tribe had exactly the same number of people saved, a fact which would be contradicted by history. The Israel of the Old Testament is here representative of the people of God of all ages as they are redeemed in Jesus Christ. These churches included Jews and Gentiles who had been drawn together by the power of the gospel. To them comes this vision, the tidings of good news. Finally, this same truth is borne out in significance of the numbers themselves.
Numbers have revelatory significance in Scripture.
The total of 144,000 is the product of 3 x 4 x 12 x 10 x 10 x 10. Three is the number of God triune. Three times four speaks of the power of the three over the four, or God's power over man, hence 12. The number ten in Scripture conveys the idea of completion, the full and perfect extension. Thus the ten plagues on Egypt formed the complete expression of God's wrath upon them, the ten virgins in the parable of Christ represented complete humanity of which five were wise and five were foolish.