How to formally calculate the surface density of layered materials such as graphene?
The C-C length in graphene is l = 0.142nm and the area of a hexagon can be calculated with the formula:

$ A = \frac{3\sqrt{3}}{2} l^2 =  0.0523nm^2$

In each hexagon, there are 2 full carbon atoms(1/3*6) so the surface density of one single layer is:

$ S_d = \frac{2*massCarbon}{A}  =  \frac{2*1.994 × 10^{-26}Kg}{0.0523× 10^{-18}m^2} = 76.26 × 10^{-8} Kg/m^2 = 7.63× 10^{-8} g/cm^2$  

If you are considering 2, 3, etc. layers than the surface density it's twice, three times etc. the surface density of the single layer.

Additional note: The distance between layers is h = 0.335 nm and therefore its density can be calculated as:

$ d = \frac{S_d}{h}  =  \frac{7.63 × 10^{-8} g/cm^2 }{0.335× 10^{-7}cm} = 2.28 g/cm^3  $  

This is very close to the experimental value I have found online says that the density of graphene is $2.267 g/cm^3$