In reactions that go until the end the stoichiometric number indicates the exact amounts of reactants and products you will get. However, this is not the case in equilibrium reactions and you will get a different amounts (concentrations) than what exhibits the stoichiometric numbers.
\ce{Na + Cl -> NaCl}
You will get in the end 1 mole of NaCl from 1 mole of Na and one mole of Cl.
\ce{2SO3<=>2SO2 + O2}
In equilibrium you will not get 1 molar of \ce{O2} and 2 molar of \ce{SO2} from 2 molar of \ce{SO3}.
Then what do the stoichiometric numbers indicate in equilibrium?
