We have a balanced chemical equation from the question that depicts the formation of water.
2H2+O2-->2H2O,
We can clearly see from the equation that, the formation of 2 moles of water molecules requires the input of 2 moles of hydrogen and 1 mole of Oxygen.
So indirectly, it tells that the moles of water molecules will be double of the moles of Oxygen molecules used in the reaction.
So if we say that 6 moles of oxygen is used and the reaction is going in such a way that hydrogen is not a limiting reactant, then 12 moles of water will be produced.
