First, you should know what is the specific surface (hkl) you are interested in. Then, you could assume that your surface is (i) not reconstructed, (ii) perfect and (iii) clean. Note that in most cases you can prepare your sample to fulfil the hypotheses (ii) and (iii) to a reasonable level of confidence. For example, you can anneal the material in vacuo to get rid of contaminants and allow surface recrystallization.
Under these assumptions, you can say that the unitary mesh of the bidimensional surface lattice is congruent to that of the exposed bulk (hkl) family of planes. Thus, you know the reference cell vectors and how many atoms the mesh contains. You can then compute the mesh area (for example, in A^2) and the atoms/area ratio will give you an estimate of the surface density. Obviously, this is just an estimate: if your surface is defective, the density will be different. Moreover, if your surface is somewhat reconstructed, you need to know the reconstruction law to perform this calculation.
If you need only a very simple and crude approximation, the average number of atoms per unit surface area should be also roughly equal to the atomic packing factor divided by an average 2-dimensional cross-section of an averaged atom present in unit cell.
If you worry about the pressure, I believe that the sample in 10^-9 MPa should have very similar cell parameters to the one in ambient conditions. The error introduced by not considering pressure should be overshadowed by the ones introduced by our other assumptions.
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