Question: how many moles of ammonia can be made?
Answer: You need 1 mole of nitrogen and 3 moles of hydrogen to make 2 moles of ammonia. If we multiply these numbers by 2, we can also say you need 2 moles of nitrogen and 6 moles of hydrogen to make 4 moles of ammonia. (And you'd have 1 mole of nitrogen left over.) And that's the answer: you'd get 4 moles of ammonia.