The total number of stereoisomers (including geometrical isomers) possible for CH3CH(OH)CH=CHCH(OH)CH3 is

The molecule has a $\mathrm{C=C}$ (E/Z possible) and two identical stereocentres (C2 and C5), so it is symmetrical. For each geometry the centres can be $(R,R)$, $(S,S)$ (a pair of enantiomers) or $(R,S)$ (meso, identical to $(S,R)$). That gives $3$ isomers for $E$ and $3$ for $Z$, so $6$ in all. The formula $2^3=8$ overcounts because the molecule has a plane or centre of symmetry in the $(R,S)$ forms.