Where the number appears
John first hears that 144,000 people are sealed on their foreheads as God's servants. He then gives a careful tribal count: 12,000 from each of twelve tribes of Israel (Revelation 7:3-4; Revelation 7:5-8). In the vision, this seal identifies those who belong to God and are under his care amid coming trouble.
Later, John sees the same number standing with the Lamb on Mount Sion. They bear the Father's name, sing a song known to them, and are described as following the Lamb faithfully (Revelation 14:1-5). The central point is clear even where details are debated: these people belong to Jesus.
And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel.
Why do people read it differently?
Some Christians understand the number literally. They see the named tribes as indicating a future group of 144,000 Israelites whom God sets apart during the events Revelation describes. This reading takes the tribal list and number in their most straightforward sense (Revelation 7:4-8).
Others understand the number symbolically. Revelation is full of vivid signs and significant numbers, and twelve is repeatedly connected with God's people. On this view, twelve times twelve times a thousand portrays the full and complete people of God, preserved for him rather than a limit on how many may be saved.
The great multitude matters too
John's next vision keeps us from treating 144,000 as the total number of people God will save. He sees a great multitude that no one can count, drawn from every nation, people, and language, worshipping before God's throne and the Lamb (Revelation 7:9-10).
That means the passage does not teach that only 144,000 will be with God. Whatever view someone takes of the number, Revelation celebrates a redeemed people far beyond human counting. Jesus' saving work reaches people from every part of the world.
After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
What the vision means for us
It is understandable to want every end-times detail settled. Yet Revelation's visions were given to strengthen believers living under pressure, not merely to satisfy curiosity. The 144,000 are pictured as people marked by God, devoted to the Lamb, and able to stand because they are his (Revelation 14:1-5).
So the most personal question is not whether you can decode every symbol. It is whether you are trusting and following Jesus. Read Revelation 7 and 14 slowly, notice both God's protection and the Lamb's worthiness, and hold your conclusions with humility where faithful Christians disagree.