Question: If all the demons were bound in the netherworld at the time they chose to follow Satan, where did the ones come from who are in the world today?
Answer: The ones “who are in the world today” are the same ones to whom you refer, the only demons of whom I have any knowledge—and that is very slim. Clearly, demons were not “all bound in the netherworld at the time they chose to follow Satan.” In fact, I don’t think any of them are bound there yet. You remember that the legion of demons complained to Jesus, “Art thou come hither to torment us before the time” (Mt 8:29)? That “time” is when they will be thus bound one day along with Antichrist, Satan, and the false prophet—but that time has not yet come.
Then what of the statement that God “spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment…” (2 Pt 2:4)? I think “hell” refers to their ultimate end, not to where they are now.
“Chains of darkness,” I believe, refers to their spiritual state rather than a condition of being bound in a particular place. That they are in “chains of darkness” means that there is no hope for them ever to know or to be delivered by the truth from their just condemnation.