How many chiral centres are in the molecule? If there are #n# chiral centres, then in principle, there are #2^n# optical isomers. In practice, you WOULD SELDOM be asked to consider a molecule with MORE THAN 2 chiral centres. (Why not? Well, because it is ridiculously difficult to represent a three-dimensional structure on two-dimensional paper).
And of course, here in the simple case of two chiral centres, there is a catch. If there are two chiral centres, we could have #"RR"#, and #"SS"# enantiomers. We could also have #"RS"# and #"SR"#, but in some systems, #"RS"# #-=# #"SR"#; such a molecule is a #"meso"# compound (look at your organic text) which is optically inactive.
Just a further tip. Suppose you have correctly represented a chiral carbon #CR_1R_2R_3R_4# as a model OR on paper. The interchange of the position of ANY two groups at the carbon, #R_1# for #R_3#, #R_3# for #R_2# etc. results in the enantiomer. Interchange again, it need not be the original substituents, and you get the enantiomer of the enantiomer, i.e. the ORIGINAL stereoisomer. Capisce? You should establish the truth of what I am saying by making your own models and assigning priority.
