(P,V,T,n) are a variables that are related to each other by pV=nRT. In a gas cylinder (e.g cooking gas) the container has a fixed volume and temperature (assuming room temperature). When it is used for the stove, the pressure drop because n drop but the other quantities remain constant. When the pressure inside is equal to pressure outside that is 1 atm and no further amount of gas can be used. At this point , there is still some gas (n) in the cylinder . We cannot possibly empty the cylinder completly as gas will stop flowing out when pressure is equal i.e 1 atm.