At constant temperature and pressure, gas volume is directly proportional to the?

P : pressure of the gas
V : volume of the gas
n : number of moles of the gas
T : temperature
R : gas constant

PV = nRT
V = (RT/P) × n

At constant temperature and pressure, (RT/P) is constant.
Thus, V = constant × n
or V/n = constant

In order words, gas volume is directly proportional to the number of moles of the gas.

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volume of the container

PV = n RT , where n is number of moles of the gas , so at constant pressure and constant temperature , gas volume is directly proportional to number of moles of the gas

P : pressure of the gas
V : volume of the gas
n : number of moles of the gas
T : temperature
R : gas constant
Z:compressibility factor
PV = znRT
V = (ZRT/P) × n

At constant temperature and pressure, (ZRT/P) is constant.
Thus, V = constant × n
or V/n = constant

In order words, gas volume is directly proportional to the number of moles of the gas(in case of ideal gas).

In case of real gases kinetic theory of gases(please read on google) is not considered .The RT/P is constant value but value of Z(compressebility factor) changes with volume for every gas(besides hydrogen and helium).Thus V~Zn.

Number of moles.
pV = nRT