For a molecule with 2 chiral centres we could have #R, R#; #S, S#; #R, S#; and #S, R#. This is #2^n# where #n# is the number of chiral centres (here #n=2#). But of course, there is a catch.
Often with these sorts of systems, the pair #R, S#, and #S, R# (which are diastereomeric with respect to #R, R#, and #S, S#) are symmetric, i.e. these are #"meso"# comounds with an internal plane of symmetry: #R,S# #-=# #S,R# upon reflection, and these are thus equivalent molecules. If you look at your text you will find a section in the chirality chapter that considers the stereoisomerism of #2,3-"dimethylbutan-1,4-diol"#. This is a relatively simple formula that generates such diastereomers.
You will note that here, only carbon stereoisomerism is considered.
