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I've found a paper Maintaining a Rogue Military: "North Korea's Military Capabilities and Strategy at the End of the Kim Jong-il Era" by Bruce E. Bechtol Jr., Angelo State University
Excerpts seem to partially confirm number of NK SOF:
The third and perhaps most ominous (to South Korea) tripod of the
North Korean asymmetric threat is Special Operations Forces (SOF).
North Korean SOF are the best trained, best fed, and easily the most
indoctrinated of all DPRK military forces. North Korean SOF has a
variety of missions - and thus a wide variety of units. These units
can be organized by brigade or battalion, all the way down to special
two or three man "teams." Most of the SOF units fall under a variety
of commands - who often work closely together during exercises or
live operations. There are units subordinate to the Light Infantry
Training Guidance Bureau (sometimes called the "11th Corps), the
Reconnaissance General Bureau, Army Corps’ and Divisions, or Korean
People's Navy/Air Force. Most official estimates place their strength
at more than 25 brigades and five independent reconnaissance
battalions, though those numbers have probably grown significantly
since 2006.
The South Korean Ministry of National Defense now places the numbers
for SOF in North Korea at around 200,000 men. General Lee also
stated that North Korean SOF “have been trained to conduct composite
operations, such as major target strikes, assassinations of important
figures and disruptions of rear areas in South Korea." High
ranking North Korean defector Hwang Jang-yop stated in testimony that
"Each North Korean special forces unit has been assigned a specific
target in South Korea, usually strategic objectives such as missile
bases and airfields. The units will be delivered to their targets by
parachute or hovercraft." Military training by SOF during the
winter of 2010-2011 was at typical high levels. According to press
reports, "Light infantry soldiers march 20 km for 10 hours with a 35
kg gear bag. On the way to the mountains, they train attacking,
ambushing, infiltrating and camping. When they arrive at the
assembly place, they would have a martial arts match between units to
have actual experience."
However, this paper as well as sources it references seems to be using "special operation forces" and "light infantry" interchangeably. See last quote above and also:
North Korea has also been able to augment the troop strength of its
SOF by converting several conventional divisions to light infantry
divisions
Also the role of NK SOF is described as infiltration and opening "second front" behind enemy lines.
So if NK SOF is just light infantry trained for opening front behind enemy lines, on US side as units with equivalent role one would have to count besides 67,000 strong USSOCOM, also 2 Airborne divisions, 2 Mountain divisions and 7 Marine Expeditionary Units. Which would put total into 170,000-200,000 not counting supporting units. Thus the part about NKSOF being largest force doesn't seem so obvious.
As for level of training there are huge discrepancies between reports, but seem that reports of "highly trained" special forces talk mostly about endurance training and hand-to-hand combat (see above). On the other hand other reports say:
North Korean soldiers are poorly fed, according to analysts and
reports from defectors, and rarely train due to scarcity of fuel and
ammunition (source)
Another source, Master thesis: "Can North Korean Special Purpose Forces Successfully Conduct Military Operations Against the United States and South Korea?", Major Samuel M. Allmond, Faculty of the U.S. Army
Command and General Staff College
The research and analysis would indicate that the quality of NK
airborne SPF training and mission preparation runs parallel with any
of the top military powers in the world to include the United States,
the former Soviet Union, China, and South Korea. Because of the
importance of their mission, research revealed the soldier of today’s
NKPA are better fed, educated, motivated, and equipped than their
predecessors who fought in the Fatherland Liberation War. NK SPF
training lasts from 12 to 24 weeks or longer depending skills being
acquired. In general, the NKPA soldier is wiry, well muscled and kept
in top physical condition by constant, strenuous physical training.
The KPA soldier is taught to be a revolutionary combatant. They are to
be strong in ideology, combat strength and determination. Above all
else, the NK SPF soldier is taught not to surrender to enemy forces
under any condition. Because of their mental and physical
conditioning, the NKPA soldier is noted for his stamina and
capabilities in all terrain and weather. Those soldiers who excel in
all aspects of their training are considered for SPF membership.
These soldiers equate to the best of the best that the NKPA has to
offer. Important to note, during research it was determined that what
they may lack in technical advantage, they appear to make up for in
stamina, commitment, and determination. The net result of the NKPA’s
SPF training system is tough, intensively trained fighter who can
travel farther and faster with more equipment and less food than
almost any other soldier.