Answer:
When Lucifer was expelled heaven, he took a third of gods angels with him. Since they are immortal and can neither die nor multiply their number has remained the same.
Since the question "How do you make demons"? is not relevant maybe you mean "How do you invoke a demon?"
The answer to this is different for every demon. Usually it requires knowing and speaking the demon's true name in the summoning. There may be all manner of rituals and possibly sacrifices involved, most of which are gruesome and/or wildly illegal.
It's also extremely dangerous. You must know how to bind your demon before summoning it or the best you can hope for is that it will kill and eat you. At worst it will drag you back to hell with it. Unless you know the individual demon's weaknesses and what will bind it, it's best to leave them alone and not mess with them.
Maybe you can try summoning a cocker spaniel. They're better looking, smell nicer, are far more people friendly and not one of them has ever tried to drag a human off to a hell dimension.