This is a bit misleading. According to Will Scott, who taught CS in North Korea, there are some 3000-5000 sites on the national intranet, but it is entirely disconnected from the internet at large. So these zones are ones that are intended for consumption by foreigners.

Source: http://motherboard.vice.com/read/how-to-teach-computer-scien... (super interesting, suggest reading it)

Usually you run NS behind anycast, so the actual number of servers can be vastly bigger. But since this is NK, maybe 5 is reasonable.