I estimate roughly around 500 small housing units from what I've seen. The smallest units are about 60*30feet in size.

For a slave labor camp, it would be easy to fit 100 people in each unit with two story bunk beds. That's already 50,000 people. The units may be more crammed than I imagine, and I may have missed miscounted these housing units. 200K sounds like an upper limit but it's not impossible.

And it's generous for you to assume that they even get to sleep alone in 'beds'

Sadly, I fear it is possible if you are squeezing 50 of them into a room designed for 5 people and feeding them tiny portions of corn meal each day.

It would have to be more densely populated than the center of Hong Kong.

This town/prison/concentration camp/detention center (or two valleys) would be lucky to have 10,000 people.

We better not let the facts get in the way of chance to bash North Korea.