The concept of Shayateen and their origin
At the time of creation of Adam, it was Satan who refused to bow before him. The Qur'an refers to 'Shayateen', a multitude of Satans. Thus there are many. Where did so many come from? Was there a whole group involved? And how do jinns multiply? Do they ever die and have graveyards? What is their sustenance? If each human being has a shaitan attached to him, is this the same alter-ego (hamzad) concept we find in some books of philosophy?
The Qur'an tells us that Iblis was the Shaitan who enticed Adam and Eve to eat from the fruits of the forbidden tree. Iblis was an individual Jinn who so to say initiated the Satanic movement. The Qur'an refers to the incident at times by using the plural form of Shaitan to show that he is not alone. His progeny accompanies him in spreading evil. It is the same style of expression which the Qur'an adopts at times for a single messenger (Rasul). When the book refers to incidents where some nations rejected their respective Rasul, it says they rejected all of them. (see 26: 105, 123, 141)
Jinns multiply somewhat the same as humans do. They marry and procreate. Of course the detailed processes must be different. They go through the same process of life and death as humans do and will be resurrected the same way as they shall be resurrected to be held accountable for their respective performances in this life. Surah al-Rahman (the 55th chapter) addresses Jinns the same way as it addresses humans in its famous recurring verse: "So which of the marvels of your Lord will the two of you (men and jinn) deny?" The Qur'an also clearly points at the fact that Jinn messengers came to guide them quite the same way as men messengers came to guide humans. (6: 130).
Of course, we don't know what they eat but quite obviously they also have arrangements of sustenance as we do. There are some references in ahadith to the fact that they eat some of the stuff we don't find worthy of eating like bones etc
The Qur'an informs us that if an individual draws away from remembering God, a Satan is appointed to dissuade him from the right path. He continues to appear to be his partner (qarin) and the human thus led astray lives in a deluded impression that he is fully guided. If the individual doesn't come back to guidance by remembering God, this companionship with the qarin continues until his death when he would realize the colossal loss he would face as a consequence of him choosing his company. (43: 36-38) I have a feeling that the concept of hamzad or alter-ego has been borrowed from this Qur'anic mention. I say it because the people who talk of hamzad also refer to it by the expression qarin. But I am not too sure of this. There is no other reference in the Qur'an to my knowledge which talks of the concept that could be referred to as confirming the common understanding of hamzad.
Let us not forget that the Qur'an clarifies that the expression Shaitan includes both Jinn and men who invite other men (and Jinns) towards the wrong path by whispering evil thoughts into their hearts. (114: 1-6)