nuclear stockpile
Refers to the total amount of nuclear weapons a country possesses.
That country's nuclear stockpile is increasing year by year.
核兵器 refers to nuclear weapons themselves, while 核保有量 refers to the quantity of such weapons held.
Compound of 核 (kaku, 'nuclear') + 保有 (hoyū, 'possession') + 量 (ryō, 'amount').