Here's an excerpt from a PDF on The history of the North Korean Army :-
The overall actual strength for the People's Army at the time
was estimated at approximately 30,000. To this figure was added approximately 170,000 trainees, some of whom were currently serving in the Police Constabulary, Fire Brigade and Coast Guard. By the end of 1948 "the armed forces had numbered some 60,000, excluding the para-military Border Constabulary and Railroad Guards. They were equipped with the weapons left behind by the withdrawing Soviet occupation troops and those shipped
into Korea to supplement the initial turnover and the ever increasing demands to meet the needs of the rapidly expanding armed force.....During 1949 the military force was doubled in strength with the addition of 40,000 conscripts, 20-22,000 Koreans-formerly in the
Chinese Communist Army, and several thousand men returning from three years
of training in the USSR to serve as cadres for the air and tank corps.
All units, including the small naval force, were given additional Soviet
equipment and their training program was intensified. Finally, preparations
for the conscription that took place in 1950 included a physical examination of all males in the younger age groups, conducted in mid-summer 1949.
In the early months of 1950 when the final preparations for the attack
were made, the tempo of military expansion increased sharply. The army was
expanded to 150-180,000 men with the addition of new conscripts and 10,000
more returnees from the Chinese Communist Army. All civilians received
basic military training, preparing the way for general mobilization.
So, along with the usual 266,000 soldiers, there were "150-180,000" new conscripts and nearly 32,000 soldiers arriving from the China. That totals up to 462,000 soldiers. That's not to mention the,
... two semi-military organizations which were drawn upon after the invasion to provide cadres for additional units.
