Four moles of ammonia combine with 5 moles of oxygen gas to make four moles of nitrogen monoxide and six moles of water vapor.
The relative numbers of moles of reactants and products are given by the coefficients in the balanced chemical equation. The coefficients will tell you any ratio of molecules of reactant and product.
If 2.0 moles of NH3 react with excess O2, then 2 moles of NO will be produced, since they are in a 1:1 mole ratio as indicated by the balanced chemical equation. Then simply run the number of moles through a conversion factor to find the mass.