Here is an explanation that I hope helps!
The hundred forty and four thousand are not being counted in Revelation 7:3-8, they are being sealed for a specific tribe of Israel (12 tribes of Israel), where all the cities for each tribe of Israel dwells within New Jerusalem, Revelation 21:10-27.
The hundred forty and four thousand that are being sealed in Revelation 7:3-8, are sealed based on Jesus’s promise in Revelation 3:12, and other scriptures.
Revelation 3:12, “Him that overcometh will I make a pillar in the temple of my God, and he shall go no more out: and I will write upon him the name of my God, and the name of the city of my God, which is new Jerusalem, which cometh down out of heaven from my God: and I will write upon him my new name.
Revelation. 21:23-24, “and the city had no need of the sun, neither of the moon, to shine in it: for the glory of God did lighten it, and the Lamb is the light thereof.” “and the nations of them which are saved shall walk in the light of it…”
The above scriptures says the nations of them that are saved. These are the multitude out of every nation, kindreds, people, and tongue, kindred, that stood before the Lamb clothes in white robes. The multitude that John was seeing in Revelation 7:9, included those that John saw being sealed who were on the earth during God’s wrath, at the sixth seal being opened, Revelation 6:12. God is sealing His servants in their foreheads. The seal in the foreheads represents the name of the city in New Jerusalem, for each tribe of the Children of Israel, which all 12 cities measured 12,000 furlongs each, Revelation 21:16, (not counting).
Important to note that God is making a point to seal his servants during the time of His Wrath upon the earth during the tribulation period, being that there are those of his servants who are still alive, but will die during the tribulations. God does not want His wrath to be against His people, Revelation 9:4, so He seals them for this purpose: so that the angels who are executing His wrath will know who they are, therefore they will not be touched. The suffering that they will receive is from the anti-christ, but not from God's wrath. So again, this seal is not counting, but is the name written on them for the name of the city of God, Revelation 3:12.
Before the angel hurt the trees, sea, and earth, Revelation 7:3 these who are being sealed were spoken about in Revelation 6:9, during the fifth seal, where other of God’s servants had been killed. White robes had been given to them also, Revelation 6:11, and they were told to rest a little season until their fellow-servants and their brothers, should be killed as they were, be fulfilled. So those servants of God being sealed in Revelation 7 would be killed during the tribulation, but they were sealed beforehand. These are those who overcome the tribulation, where God writes upon them the name of the City of God, which is New Jerusalem (measures 144,000 cubits), Revelation 3:12.
Another example of the servants of God that were sealed in Revelation 7:3, is during the Fifth trumpet, where God tells the angel to not hurt the grass or any green thing, and to only hurt those men which have not the seal of God in their foreheads, Revelation 9:4. Those who God did not want the angle to hurt in Revelation 9:4, are the same saints in Revelation 7:4-8 who have been sealed. Again to help people see that this is not counting in Revelation 7:4-8, but an actual seal in the foreheads of God’s servants, we have to look at what Jesus said in Revelation 3:12 again. And in Revelation 3:5, Jesus makes it clear that those who overcome will received white raiment (robes), and He will not blot their names out of the book of life.
One of the Elders (I believe these are those resurrected from Abraham’s Bosom when Jesus ascended, and these Elders are already in Heaven) made a point of pointing out those who come through great tribulation and had washed their robes and made them white in the blood of the Lamb, Revelation 7:13. These that came out of the tribulation serve God day and night in the Temple, Revelation 7:15-17, In addition to the great multitude that John saw, Revelation 7:9.
Note: The opening of the first seal by the Lamb to the sixth seal, encompasses the beginning of God’s wrath to the end where God will wipe away all tears. At the sixth seal you see the Lamb and God in the Temple, and you see the angels around the throne, there is no more hunger, no more thirst…," Revelation 7:16-17.
The seals, trumpets, and vials which are the wrath of God coming from heaven upon the earth, are all describing various views of the same tribulation event from beginning to end. What’s happening is each one is giving different accounts of events for the same period.
The Hundred Forty and Four Thousand standing on Mount Sion with the Lamb, has the Lamb’s Father’s name written on their foreheads, Revelation 14:1. These are the same sealed in Revelation 7:3-8, and spoken about in Revelation 3:12.
The 144,000 standing on Mount Zion, Revelation 14:4 are called virgins.
“For they are virgins” does not mean men who never had a relationship with a woman. It means those who are saints in Christ, and who are to be married to the Lamb:
2 Corinthians 11:2, for I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ.
Matthew 25:1, then shall the kingdom of heaven be likened unto ten virgins, which took their lamps, and went forth to meet the bridegroom.
Matthew 25:6, and at midnight there was a cry made, behold, the bridegroom cometh; go you out to meet him.
Matthew 25:7, then all those virgins arose, and. Trimmed their lamps.
Matthew 25:11, and afterwards came also the other virgins, saying, lord, lord, open to us.
Revelation 19:7, let us be glad and rejoice, and give honor to him: for the marriage of the lamb is come, and his wife hath made herself ready
Revelation 21:2, And I john saw the holy city, New Jerusalem, coming down from heaven, prepared as a bride adorned for her husband.