Amount of gas
The number of molecules (Nmolecules - proportional to the amount or mass of the gas) in this simulation is much, much less than any sample of gas that would be handled in the real world.
This allows the simulation to run at reasonable speed, and helps you view the properties and behaviour of the gas as arising from individual molecules bouncing around a box.

Note:
Sometimes we measure a quantity of a substance by the number of particles. The unit of measurement often used is the mole - comprising 6.022 x 10²³ molecules of the substance (with a mass of n grams - where n is the substance's molecular mass). Moles are most often used to measure gases (rather than liquids or solids), because equal number of moles of any gas (at the same temperature and pressure) occupy the same volume. At 0^oC and atmospheric pressure, one mole of any gas occupies 22.4 Litres.