The NKPA started the war with 10 divisions, but added more to the invasion force.The NKPA was a tight 10 division army. Well trained. They had wanted another tank division but fraternal relations only went so far. They lacked strategic reserves. They had also been fooled by their politicians that a revolutionary movement in the south was capable of directly seizing major objectives independently
The early NKPA advances were an example of the culminating point of victory. They burnt through their supplies and reserves historically. They were highly motivated well trained and ready, but fragile.
According to the first official census in 1949 the population of North Korea numbered 9,620,000, and by mid-1950 North Korean forces numbered between 150,000 and 200,000 troops, organized into 10 infantry divisions, one tank division, and one air force division, with 210 fighter planes and 280 tanks, who captured scheduled objectives and territory, among them Kaesong, Chuncheon, Uijeongbu and Ongjin. Their forces included 274 T-34-85 tanks, 200 artillery pieces, 110 attack bombers, and some 150 Yak fighter planes, and 35 reconnaissance aircraft. In addition to the invasion force, the North had 114 fighters, 78 bombers, 105 T-34-85 tanks, and some 30,000 soldiers stationed in reserve in North Korea. Although each navy consisted of only several small warships, the North and South Korean navies fought in the war as sea-borne artillery for their armies.
During the course of the fighting July-September the NKPA suffered the following estimated casualties.
By 1 October 1950, the UN Command repelled the KPA northwards past the 38th Parallel; the ROK advanced after them, into North Korea. MacArthur made a statement demanding the KPA's unconditional surrender. Six days later, on 7 October, with UN authorization, the UN Command forces followed the ROK forces northwards. The X Corps landed at Wonsan (in southeastern North Korea) and Riwon (in northeastern North Korea) on 26 October, but these cities had already been captured by ROK forces. The Eighth US Army drove up western Korea and captured Pyongyang on 19 October 1950. The 187th Airborne Regimental Combat Team made their first of two combat jumps during the Korean War on 20 October 1950 at Sunchon and Sukchon. The mission was to cut the road north going to China, preventing North Korean leaders from escaping from Pyongyang; and to rescue US prisoners of war. At month's end, UN forces held 135,000 KPA prisoners of war. As they neared the Sino-Korean border, the UN forces in the west were divided from those in the east by 50–100 miles (80–161 km) of mountainous terrain. In addition to the 135,000 captured, the KPA had also suffered some 200,000 men killed or wounded for a total of 335,000 casualties since the end of June 1950, and had lost 313 tanks (mostly T-34/85 models). A mere 25,000 KPA regulars retreated across the 38th Parallel, as their military had entirely collapsed. The UN forces on the peninsula numbered 229,722 combat troops (including 125,126 Americans and 82,786 South Koreans), 119,559 rear area troops, and 36,667 US Air Force personnel.
Using these as rough figures if the North invaded the South with 200,000 men in June, and in October had 25,000 left, after suffering 335,000 casualties they somehow picked up 160,000 reinforcements. So where did they come from? I've read estimates that 30,000 POW's claimed they were South Koreans forced to fight for the North, and say another 10,000 of these were KIA, the other 100-120,000 must have been new drafts from the North, or ethnic Koreans from China, so a lot of reinforcements went South following the initial invasion.
Once back in NK they continued a fighting withdraw, linking up with the PLA, and joining them in the First Phase Offensive. So after suffering operational loses in the 90% range, they still continued to offer effective organized resistance, and then counterattacked, so just how fragile were they?