It is very difficult to calculate how many faces each pot can cover, as it varies depending on how thickly you apply the paint and how much of the face you want to cover.   

As a guide:  

1 x 18ml pot can cover anywhere from 50 to 100 full faces. 
1 x 30ml pot can cover anywhere from 80 to 160 full faces. 
1 x 75ml pot can cover anywhere from 200 to 400 full faces.