The key parameter is the surface site density. You can determine this with the following equation:
surface site density [mol/m2] = n(desorbed) [mol]/(S(BET)[m2/g]*m(adsorent)[g]*surface coverage [-])
So you need the amount of desorbed probe molecule which can be done by TPD experiments. Furthermore you must assume a surface coverage. In general at low adsorption temperatures you can assume a nearly complete coverage by your probe molecule (i.e. surface coverage ca.(!) 1).
If you deduced the surface site density you can at least estimate the approx. surface coverages with the equation stated above for any loadings. Though to get a more accurate surface coverage you'll need a kintic model for your adsorption process.