Here's Revelation 7:4 from the New World Translation of the Bible
And I heard the number of those who were sealed, a hundred and forty‐four thousand, sealed out of every tribe of the sons of Israel:
In the article "Great Crowd" in the Watchtower Biblical/theological encyclopaedia Insight on the Scriptures, we find:
The apostle [Paul] wrote: “He is not a Jew who is one on the outside, nor is circumcision that which is on the outside upon the flesh. But he is a Jew who is one on the inside, and his circumcision is that of the heart by spirit.” (Romans 2:28, 29) Why, too, would there be no mention of any ‘sealing’ of the Gentile Christians in this divine vision? And why would not the Gentile Christians be able to master the new song sung by the 144,000? (Revelation 14:3) It thus seems clear that the 144,000 sealed ones are of spiritual Israel, not fleshly Israel—hence include both Jewish and Gentile Christians.—Galatians 6:16.
The 144,000 sealed ones are of spiritual Israel. In other words, adopted into one of the tribes.
Here's a further explanation from the article "Do All Faithful Christians Go to Heaven?" in the 1 June 2011 issue of The Watchtower magazine.
So, too, with this heavenly Kingdom. Jesus, the Lamb of God, rules with 144,000 co-rulers “bought from among mankind.”—Revelation 14:1, 4, 5.
True, 144,000 is a small number compared with the number of all faithful ones, past and present. But this is understandable because the 144,000 are resurrected to heaven for a specific sacred assignment of work. If you were to build a house, would you contract all the skilled builders in your area? No. You would need only as many as were required for the job. Similarly, not all faithful ones are chosen by God for the unique privilege of ruling with Christ in heaven.