Why does Satan collect souls that are subservient to him according to mainstream Christian scriptures? Pardon my ignorance, I'm by birth a non-christian and just trying to learn...keeping up with the philosophy of this stack-exchange forum.
According to Christian, Gnostic/Book of Q or other Abrahamic scriptures,
is there any description on Why Satan collects souls that are made subservient to him later on? To what purpose does it serve? Is it to fight with the Second coming of Jesus or to fight ultimately with the Father/God? Do they remain subservient to him after they are dead? Is there are reference to the number of souls he needs for that purpose?
A better question might be “Is there any biblical basis for thinking that Satan collects souls that are subservient to him?”  That’s because there is no such teaching according to mainstream Christianity.
The idea that Satan “collects souls” is neither biblical nor Christian.  The Bible tells us that every living soul belongs to God” (Ezekiel 18:4) and that only God has the power to kill the soul (Matthew 10:28).  Jesus also said that there is an eternal fire prepared for the devil and his angels (Matthew 25:41) and that Jesus has the keys of death and hades (Matthew 16:18).   Hebrews 9:27 informs us that “man is destined to die once and face judgment”.  This is God’s judgment, not Satan’s. Wherever the souls of persons who have physically died end up is entirely according to God’s will, not Satan’s.

Nothing in all creation is hidden from God’s sight.  Everything is uncovered and laid bare before the eyes of Him to whom we must give account” (Hebrews 4:13).

There are some denominations, however, that appear to subscribe to the power of Satan and his demons with regard to having control over the souls of the dead:

‘Aerial toll houses’ refer to a teaching held by some Eastern Orthodox saints and Eastern Orthodox Christians about the immediate state of the soul after death. It holds that &quot;following a person's death the soul leaves the body, and is escorted to God by angels. During this journey the soul passes through an aerial realm, which is inhabited by wicked spirits (Ephesians 6:12). The soul encounters these demons at various points referred to as toll-houses where the demons then attempt to accuse it of sin and, if possible, drag the soul into hell.&quot;[1]
In some forms, the teaching is taught in hagiographical and other spiritual texts from quite early in the history of the church, but it has never been formally promulgated by any ecumenical council.[1] A number of the Orthodox saints, modern elders and theologians have openly endorsed it, but some theologians and bishops, starting from the last century, have condemned it as heretical and gnostic in origin.[2] The content is somewhat similar to that propounded by John Climacus in The Ladder of Divine Ascent. Some say that given the amount of fear that comes along with the teaching, the love of Christ becomes misunderstood and is forgotten, but others suggest that fear is perfectly natural and salvific for Orthodox Christians.  Source: https://en.wikipedia.org/wiki/Aerial_toll_house

Another source of “misinformation” comes from ‘The Divine Comedy’, written by Dante Alighieri between 1308 and 1321. It is written in the first person as Dante describes his imaginative journey through the three realms of the dead:
Inferno (hell); Purgatorio (Purgatory); and Paradiso (heaven).  Dante describes hell as comprised of nine concentric circles, representing an increase of wickedness, where sinners are punished in a fashion befitting their crimes.  At the bottom of hell, Satan waits to gorge on the miserable creatures who reach him.

Dante’s literary vision of hell is depicted by Botticelli in his painting Map of Hell as a subterranean funnel of suffering—a wretched underground landscape of fire, brimstone, sewage, and monsters, with Satan himself waiting at its core. It’s all very disturbing, and effective as a work of art, but it is based on the imaginations of men, not the Word of God.
In Dante's Inferno, Satan is portrayed as a giant demon, frozen mid-breast in ice at the center of Hell.  As opposed to the popular conception of the era, which viewed Satan as an all-dominating beast of Hell, Dante gives the portrayal of Satan as simply another victim of Hell's tortures. He places Satan trapped within the ice, stripped of voice and power and thus sets forth a new conception of who and what Satan is.  Source: https://en.wikipedia.org/wiki/Dante%27s_Satan

These views do not represent mainstream Christian beliefs about what happens to the soul at death.  Jesus said:

Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels (Matthew 25:41).

God and Jesus are in control.  End of.