The other mention of 144,000 occurs in Chapter 7 (4-8), which refers specifically to members of each of the 12 tribes of Israel (12,000 per tribe):
Then I heard the number of those who were sealed: 144,000 from all the tribes of Israel. From the tribe of Judah 12,000 were sealed, from the tribe of Reuben 12,000, from the tribe of Gad 12,000, ...
This is definitely not what is referred to here, since - as you quote - the multitude listed were virgins (v.4). This was explained in the earliest complete commentary ever written on Revelation, by Andrew of Caesarea (563-637):
We believe that these, after the aforementioned twenty-four elders, are superior to the rest on account of both virginity [v.4] and blamelessness in tongue and hand [v.5], after the appearance of Christ possessing splendor in virtues through which they are taught the new song, the song which is unknown to the many, not only in the present life but also in the
The passage you quote may be an allusion to 2 Esdras 2:42-48:
I, Ezra, saw on Mount Zion a great multitude, which I could not number, and they all were praising the Lord with songs. In their midst was a young man of great stature, taller than any of the others, and on the head of each of them he placed a crown, but he was more exalted than they. And I was held spellbound. Then I asked an angel, “Who are these, my lord?” He answered and said to me, “These are they who have put off mortal clothing and have put on the immortal, and they have confessed the name of God; now they are being crowned, and receive palms.” Then I said to the angel, “Who is that young man who places crowns on them and puts palms in their hands?” He answered and said to me, “He is the Son of God, whom they confessed in the world.” So I began to praise those who had stood valiantly for the name of the Lord. Then the angel said to me, “Go, tell my people how great and many are the wonders of the Lord God which you have seen.”2
The number is almost certainly symbolic, though the precise meaning is not spelled out here as it is in 7:4. Regarding the meaning of the numbers, commentator Lawrence Farley writes:
As is usual in apocalyptic literature, the numbers here have symbolic significance. One thousand is the number of vast plurality. To say that God owns “the cattle on a thousand hills” (Ps. 50:10 [LXX]) is to say that He owns the cattle on all the hills. Further, twelve is the number that denotes completeness— such as in the twelve tribes of Israel, the twelve apostles.3
That the virgins follow the Lamb wherever he goes suggests that the passage refers to members of the Church:
1 Peter 2:21
For to this you have been called, because Christ also suffered for you, leaving you an example, that you should follow in his steps.
1. Translated from the Greek in E. Constantinou, Andrew of Caesarea and the Apocalypse in the Ancient Church of the East (Ph.D. thesis, Université Laval, 2008).
3. The Apocalypse of Saint John: A Revelation of Love and Power