There isn't clear official information, or I cannot find on the web on how many domestic flights are there between places in North Korea, but I could find some articles, Tour Company page and blog post about domestic routes in North Korea.
https://en.wikipedia.org/wiki/Transport_in_North_Korea#Air_transport
https://en.wikipedia.org/wiki/List_of_airports_in_North_Korea
Wiki states basic situation of North Korean Air Transportation.
As I reckoned general North Koreans haven't got freedom of movement, whoever wants to travel in the country should obtain travel permission. Which in Airlinsim that could may be effect Passenger Demand generation between places in North Korea.
https://en.wikipedia.org/wiki/Air_Koryo_destinations
This wiki page states destinations for Air Koryo which also lists domestic and international destnations.
http://uritours.com/blog/2015-air-koryo-flight-updates
On the bottom of this page, It lists some flight schedules for domestic destination form FNJ.
http://uritours.com/air-koryo
International flight schedules for Air Koryo.
http://www.reuters.com/article/us-northkorea-tourism-idUSKBN0F80WC20140703
This article states approximate estimated figure of international arrival for tourism purpose to North Korea.
237000 Chinese, 6000 Westerners.
It looks major domestic destinations seem to be these.
Chongjin Orang Airport, IATA: RGO, ICAO: ZKHM
Wonsan Kalma Airport, IATA: WOS, ICAO: ZKWS
Samjiyon Airport, IATA: YJS, ICAO: ZKSE
Hamhung Sondok Airport, IATA: DSO, ICAO: ZKSD
Haeju Airport, IATA: HAE
Sinuiju Airport
Chongjin Orang Airport serves city of Chongjin, city of Rason which is few of foreign capital driven economic developing area of North Korea, and intended second international hub for the country.
Wonsan Kalma Airport serves city if Wonsan, also a gateway to one of North Korean natural reserves, Mount Kumgang, famous among tourists. It seems that It also has got new terminal built to receive international passengers.
Samjiyon Airport serves manly for another North Korean natural reserves, Paektu Mountain, which is also famous destination for tourists.
Hamhung Sondok Airport serves city of Hamhung.
Haeju Airport serves most densely populated pain farming region of North Korea where much of North Korea is sparsely populated for its mountainous terrain.
Sinuiju Airport serves city of Sinuiju which borders Chinese city of Dandong, and main land border and port between North Korea and China.
There are each airports detail on Wiki pages.
Cheers.
:D