You are both correct and wrong at the same time. Two atomic bombs were dropped on Japan, but at seperate locations, Hiroshima and Nagasaki. So Hiroshima 2 is something that could happen, but hasn't yet. The two bombs killed an rough average of around 100,000 each. When compared to the current human population, we'll say its 7 billion to make the math easy, each bomb would only kill about .000014% of the population. Therefore, it would take 70,000 atomic bombs to eradicate the world, although the world is not evenly populated, so there would be much deviation.
With all that being said, a hyrdogen bomb is roughly 500 times the strength of a nuclear bomb. If you substitute hyrdrogen bombs for atomic bombs, you would only need 140 hyrdrogen bombs.