Let’s just outline the specifics of the casualty estimates for a war with North Korea in the latest Congressional Research Service (CRS) report dated 27 October 2017: https://fas.org/sgp/crs/nuke/R44994.pdf
On page 18:
Even if the DPRK uses only its conventional munitions, estimates range from between 30,000 and 300,000 dead in the first days of fighting, given that DPRK artillery is thought to be capable of firing 10,000 rounds per minute at Seoul. One observer states
Estimates are that hundreds of thousands of South Koreans would die in the first few hours of combat–from artillery, from rockets, from short range missiles–and if this war would escalate to the nuclear level, then you are looking at tens of millions of casualties and the destruction of the eleventh largest economy in the world.
It does not appear that CRS has done any independent analysis of this issues. Its sources in the footnotes are articles from Reuters, New York Times, CNN, NAPSNet Special Reports and GlobalSecurity.
And on page 3:
Should the DPRK use the nuclear, chemical or biological weapons in its arsenal, according to some estimates casualty figures could number in the millions.