While Halden is a maximum security prison, it's also highly focused on rehabilitation, so it's not necessarily where the worst offenders are (Breivik is at Ila for instance, which can hold 124 people).
Halden is actually a fairly big prison by norwegian standard I think, the capacity of the whole Norwegian prison system is a bit below 4000 (77 per 100000 citizens, the current incarceration rate is 72 per 100000 according to wikipedia).
Most people with roommates still want a private room to retreat to.
Here in Finland, the largest prison in the country can accomodate 299 inmates. I imagine Norway's situation is roughly similar.
[0] http://www.kriminalomsorgen.no/finn-fengsel.237849.no.html
I mean I had to share some rooms for some weeks with different people, it was one of the worst experience of my life: different behaviors, idea of "respecting personal spaces", idea of "maximum noise allowed", habits at night, hygiene, etc... It is really something stressful. In a prison, when you risk to end up with people you probably won't like at all, is far better to have a personal room to stay for the big part of the time.
hmmm...Private rooms could be part of the problem. Isolation can create psychosis. Maybe 12 hours a day is good enough, but I think total isolation should be avoided whenever possible, or let prisoners choose whether or not to be isolated. Integrating prisoners saves money and makes then happier. But these are probably among worst offenders given that there are only 250.