From the article you linked, "ITER is hoping to produce 500 megawatts over 1,000 seconds from just 50 megawatts of input power and 0.5 grams of hydrogen fuel."

From the article you linked, "ITER is hoping to produce 500 megawatts over 1,000 seconds from just 50 megawatts of input power and 0.5 grams of hydrogen fuel." (My emphasis.) The half-gram of fuel is to run for minutes, not months. With optimistic assumptions, complete fusion of one gram of He-3 (with deuterium) can produce 1/3 * 6.02 * 10^23 * 18.354 * 4.4505 * 10^-20 = 163913 kWh of primary (thermal) energy. If world primary energy demand is 18 terawatts, that's 18 * 10^9 * 24 * 365 = 157680000000000 kWh annually. 157680000000000 / 163913 = 961973729 grams (962 metric tons) of He-3 per year to cover all primary energy demand.