This is a interesting excerpts from a study by Ekkehardt Mueller of the SDA Biblical Research Institute.

"Introduction
The number 144,000 is one of several apparently cryptic numbers of Revelation that has caused much speculation. Generations of Christians including Adventists have studied this symbol. In the past, the Biblical Research Institute has already published an article on the 144,000 and the Great Multitude by Beatrice Neill.1[1] The present article is an attempt by its author to deal with the biblical information. When addressing this topic the following questions must be addressed: What kind of group are the 144,000? When do they appear? In which relation do they stand to the great multitude and the remnant? Is it a literal or figurative number?

I. The 144,000 as a Group
In Scripture the 144,000 appear under this designation only twice, namely in Rev 7 and in Rev 14. They are a specific group of human beings which maintain a special relationship with Jesus. In Rev 14:1 they are standing with the Lamb, Jesus, on Mt. Zion. Those, who have been condemned and have been persecuted in chap.13, now triumph with the Lamb.1[2] Instead of the mark of the beast on their foreheads or their hands these people bear the name of the Lamb and of the Father on their foreheads. They belong to God. He keeps them. They resemble Him. They sing a new song, a song of their personal experience, which they went through in the final battle between truth and error, God and Satan. Even though they were unable to buy or sell (13:17), Jesus has bought them. Salvation was expensive. It cost Jesus' life. The 144,000 have not defiled themselves with women. They are virgins. This means that they have not entered into a relationship with false religion or they have separated themselves from it.1[3] They follow Jesus every step of the way (cf. John 10:27-28) and are transformed through God's grace. As the first fruits they are a special group of children of God. According to the context of Rev 14 they have accepted the three angels' messages and may also have proclaimed them. In Rev 14:12 the saints are another name for the 144,000. These saints are characterized by endurance/patience, keeping of the commandments, and faith of Jesus. Thus, Rev 14 furnishes information on their character and behavior. Rev 7, however, contains little in this respect. We hear that the 144,000 are God's servants who are going to be sealed (Rev 7:3). The seal of God on their foreheads points to the fact that they are God's property and that they are at least protected from apostasy.1[4]

II. The Time of Their Appearance and Ministry
Although the 144,000 are already presented on Mt. Zion, their acceptance of and obvious involvement in the proclamation of the three angels' message, which takes
place immediately before Christ's Second Advent, make it clear that they are God's end time people. The same is true when we take a look at Rev 7. The sixth seal contains the heavenly signs of Jesus' Second Coming (6:12-14) and the Day of Lord, which is the Second Advent (6:14-16). The chapter ends with the question: "Who can stand before it?" (6:17). The previous verses (6:15-16) portray people who are not able to survive the day of the wrath of God and the lamb. On the other hand, Rev 7 points to people who are able to stand. Thus, Rev 7 responds to the question of 6:17 telling us that the 144,000 will be able to stand (7:1-8). The great multitude (7:9-17) is already depicted as serving God in heaven, in his heavenly sanctuary before his throne (7:15). The 144,000 are found in connection with the Second Advent prior to the time when the saints will be with God in heaven. Again as in Rev 14, the 144,000 of Rev 7 are God's end time people, apparently those who will be alive when Jesus returns to take his children home.

III. The 144,000 and the Great Multitude
Opinions vary on the relationship between the great multitude and the 144,000. The two groups could be distinct from each other; the 144,000 could be a part of the great multitude; or the 144,000 and the great multitude could be one group appearing under different names.1[5] The arguments for the latter option seem to be the most convincing. Obviously, the 144,000 and the great multitude, which will be found before the throne of God in his sanctuary, refer to the same group.1[6]
1. In Rev 5 God has a scroll in his hand which initially nobody is worthy to open. While John weeps, an elder tells him (5:5) that the Lion of the tribe of Judah, Jesus, has overcome and is able to open the scroll. John hears about the lion1[7] but when in 5:6 he takes a look, he does not see a lion but a lamb. Jesus the Lion is Jesus the Lamb.1[8] This phenomenon is found in the introductory scene of the seal vision. It is repeated in the same vision, in chapter 7. In 7:4 John hears the number of the sealed, but in 7:9 he sees that the 144,000 that were sealed form a great multitude. The 144,000 and the great multitude are the same group portrayed from different perspectives. The first designation is a symbolical term, the latter describes reality.
2. The answer to the question of who will be able to stand (6:17) is provided by the entire seventh chapter. Both depictions of a group, the 144,000 and the great multitude are those who are able to stand and point to the same group.
3. The 144,000 are introduced as an immediate answer to the question of 6:17. They are not further described, and the consequence of being able to stand on the great day of the wrath is not shown in Rev 7a, but only in Rev 7b. However, the great multitude receives a more detailed description and is depicted as standing before God. The same term "to stand" is used in 6:17 and 7:9.
4. The 144,000 as well as the great multitude have to go through difficult times. The 144,000 are sealed before the winds blow and have to stand the succeeding difficulties. The great multitude has come out of the great tribulation.
5. The 144,000 are God's end time church on earth. The great multitude is the end time church in heaven. The 144,000 are God's militant end time church. The great multitude is God's triumphant end time church.
6. The 144,000 are the fullness of God's end time church, twelve times twelve times thousand. This number reminds us of the twelve tribes of Israel and the twelve apostles of the Lamb (21:12,14). The number thousand may point to a military unit in the Israel of old (Num 31:4-6). Thus, the 144,000 represent the church militant of the end times.1[9] The great multitude is the consummated end time church. The information on the great multitude complements what was lacking with the 144,000. The sealing of the 144,000 would be incomplete if it would not lead to the final consummation as depicted by the experience of the great multitude.
7. The 144,000 are "servants of our God" (7:3). The great multitude "serves" God (7:15). Both Greek terms are used for the same group in 22:3.
It seems to be best to understand the 144,000 and the great multitude as the same group viewed from different perspectives. The great multitude does not comprise the redeemed ones of all ages. This is not the focus in Rev 7, nor is it denied that there will be saved ones from all ages. In fact this is implied in 14:4 by calling the 144,000 the first fruits. There is going to be a universal harvest of the redeemed ones from all ages past.

IV. The Great Multitude and the Remnant
The central vision of Rev focuses in a special way on the church. Whereas Rev 12 starts with the early church, stresses the church during the medieval period, and introduces the end time remnant. The following two chapters focus specifically on the end time people of God and their fate. Rev 12:17 talks about the remnant. Rev 13:1-10 follows and mentions the saints which are not involved in universal false worship. Rev 13:11-18, knows about a group not mentioned by a specific name that does not accept the mark of the beast and does not worship the beast or its image. In spite of the universal boycott and the death decree aimed at God's faithful remnant, there is a group of survivors, who live with Jesus, the 144,000 of Rev 14. Obviously the remnant, the saints, those who do not receive the mark of the beast and do not worship the beast and its image, and the 144,000 are the very same group. The passages are linked linguistically and structurally.1[10] The phrase "to make war with/against" is found in Rev 12:17 and in 13:7. In 12:17 the war is waged against the remnant. In 13:7 war is waged against the saints. Apparently, the remnant of 12:17 and the saints of 13:7 describe the same group.1[11] Whereas the sea beast reflects the beasts of Dan 7, the saints of Rev 13 remind us of the saints found in Dan 7.1[12] Although the saints in Rev 13 seem to describe the end time remnant, nevertheless, they are the continuation of the saints throughout Christian history as the remnant is the offspring and continuation of the woman of Rev 12. Rev 12:17, Rev 13:10, and Rev 14:12 are interwoven and describe the main characteristics of the remnant appearing under different names.
Rev 12:17 Keep the commandments Testimony of Jesus
Rev 13:10 Patience Faith
Rev 14:12 Keep the commandments Patience Faith of Jesus...


VI. A Symbolic Number?
It is now quite clear that the 144,000 must be understood in a symbolic way rather than in a literal way. The immediate context is clearly symbolic (7:1-3) mentioning the four corners of the world, the four winds of the earth, sea, earth, trees, and the seal of God. But also the extended context, namely Rev 6 is largely symbolic, for instance, the apocalyptic riders and the martyrs under the altar. The parallel passage in 14:1-5 must be understood symbolically. It tells us that the 144,000 "have not defiled themselves with women." They are "virgins" and follow the "Lamb." The group does not consist of unmarried men only. The term "woman" is a symbol in Rev as is the term "Lamb." Therefore, symbolic language is also to be expected for 7:4-8. The number is symbolic?12 times 12 times 1,000?and points to the fullness of the people of God. The enumeration of tribes is very unusual. The tribe Dan is missing, whereas Manasses should already be contained in Joseph. Ephraim is not mentioned, however, Levi is counted.1[17] Judah is found in the first place and Benjamin in the last, forming a parenthesis which encompasses the tribes of the Northern Kingdom.1[18] Such a list is not found elsewhere in Scripture. Most of the twelve tribes no longer exist today. Therefore, it is hardly possible to find 144,000 literal Israelites according to their tribes as listed in Rev 7 forming God's end time people. However, the NT knows descendants of Abraham that are not literal descendants (Rom 4:11-12) as well as a spiritual Israel (Rom 2:28-29; Gal 6:16).."

See the whole study Here.