When an atom/molecule is adsorbed on a surface it alters the electronic environment in the vicinity of the adsorbption site. The effect of this will be a change in the electronic density and thus a change in the potential from the charge distribution. When you go far enough away from the adsorbtion site one will expect the local density and potential to look like the pristine interface - but how you have to go away depends on how the material responds to the adsorbant. There will be multiple effects that determine this: how much the geometry of the interface atoms actually change (nearby atoms will get pushed/pulled) and how much the electrons screen the adsorbant (dielectric properties). Depending on the material, these effects can be short range or long range.
When you do a supercell calculation, it is still a periodic crystal, i.e. you repeat the adsorbant every 3 or every 6 or so unit cells. So you create artificial system with repeated adsorbants with a density/concentration of adsorbants which is typically higher that the system you are trying to model. If you want to model a single isolated absorbant you have to make sure that the absorbants can safely be regarded as isolated, i.e. that the distance between them is longer than the above described effects. The only way to ensure that is to converge adsorbtion energy with respect to the supercell size. So you have to do adsorbtion energy calculations of increasing supercell sizes, e.g. 3x3, 4x4, 5x5, 6x6, 7x7, 8x8, ..., until the energy changes less that some threshold that you consider negligible.
