How many mols CO in this equation : #2CO + O_2 -> 2CO_2#?
You have 2 moles of carbon monoxide in that chemical equation.
In a balanced chemical equation, the stoichiometric coefficients express the number of moles of each species that takes part in the reaction.
They tell you how much of each rectant is needed and how much of each product is formed by a reaction.
A stoichiometric coefficient is the number that comes before a compound's chemical formula in a balanced chemical equation.
In your case, you have
Notice that you only write coefficients that are bigger than 1 in the balanced chemical equation.