The central quantity of interest in adsorption measurements is the amount of gas adsorbed on the surface. This is often given in "per area" units, such as mol/${\displaystyle m^{2}}$ or molecules per square nanometer. In the case of a porous material, in which the surface area is distributed throughout the interior of the sample, adsorption is often quoted in "per volume" or "per mass" units, where the volume or mass refers to the total volume of the material.
For adsorption in porous materials, where the total surface area for a sample can be large (say, 100 ${\displaystyle m^{2}/g}$), adsorption measurements are most often made using a gravimetric apparatus. Basically, you expose a sample of porous material to a known pressure of gas, and measure the mass of the sample+adsorbate. By scanning the pressure up and down at fixed temperature, you obtain adsorption and desorption isotherms.
For low-surface-area samples as used in surface science and studies of precious-metal catalysts, the total amount of adsorbed gas is very small. In such cases specialized techniques and ultra-high-vacuum equipment must be used to measure adsorption. For instance, on a 1 cm${\displaystyle ^{2}}$ piece of crystalline platinum, a complete monolayer of carbon monoxide contains about ${\displaystyle 1.3\times 10^{15}}$ molecules, which is only 2.1 nanomoles. That's not a lot.