If you forget to take a dose, take it as soon as you remember. Then take your next dose at the usual time. Do not take two doses to make up for the missed dose.
For controller inhalers with dose counter, check the dose counter and change the canister once the dose counter indicates zero  doses.
For controller inhalers without dose counter, mark the date of opening and based on the dosing regimen, throw away when the doses are used up.