Does it mean mass? If so, wouldn't Pressure/Temperature basically represent its density?
Sounds like constant amount of substance (as in the dimension or physical quantity, "mol" being it's unit). Could also mean mass, but mass is often not strictly constant. If you add energy (e.g. heat), the mass of the gas increases slightly via E=m*c2.
Could you provide more context?
In thermodynamics physicists and engineers use the concept of 'control volume' that specifies a bounded region of space in which thermodynamic properties of a gas are analyzed.
The boundary of the control volume can be used to either isolate or otherwise define a specific flux of energy and/or matter to/from the control volume.
I believe the "fixed amount" of gas refers to a situation where the control volume has zero mass flux through the boundary, but not necessarily zero energy flux.