There may well be more demons than people.
Nor would it be necessary for a demon to be assigned one-to-one with humans anyway. Demons are not omnipresent, but they are fast. Have you ever watched one of those old-time plate spinner acts? The guy starts dinner plates spinning at the tops of poles. He first starts one plate spinning, then starts another, then another, then another. By that time, the first place is slowing down and about to fall off the pole, so he rushes back and spins that plate back up. He starts more and more plates spinning, and just runs back and forth keeping them all going.
Demons can easily do the same thing. They have billions of hours of experience in watching human behavior. They have far more than enough time watching your behavior, and they probably watched your parents and ancestors. They know what buttons to push, because they watched your buttons being installed. Demons are able to set people spinning and return as often as necessary to keep them spinning.
The only way out of that is through the Holy Spirit. We in ourselves can never catch a demon by surprise, but the Holy Spirit can. Only by carefully following the Holy Spirit can we ever make a demon say, "Didn't see that coming."