How often do people have demonic encounters?

It depends on the demon's motive. If the demon has a grudge on you, he will follow you and try to torture. If the demon is a lost spirit, wanting to be alone as he roams, he will encounter you as long as you're in his territory. If the demon is a slave of Hell, you are supposed to be dead but somehow avoided it...

One interesting thing to note is that only people who believe in demons and demonic oppression/possession ever encounter them. Non-believers find realistic answers to explain seemingly odd things that happen in their lives rather than relying on supernatural explanations.