I've had several different apartments in Korea over the years and never really had a major problem with bugs. I'm not saying there haven't been bugs, but the number and type haven't been anything like a serious infestation or problem.
I don't know what the actual percentage is of foreign English teachers who are placed in apartments that have bug problems. I'd have to say based on how many teachers I've heard talk about it as a problem that it'd be something like 25% or so. The majority of apartments in Korea will from time to time have ants, cockroaches, and other critters appear to one degree or another--but in terms of them being a chronic presence and problem relative to apartment life in Korea I'd say 25% sounds accurate in my experience of hearing stories from teachers.
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Bugs and Apartment Life in South Korea - A giant cockroach visited my girlfriend today . . . no, really.