The total number of stereoisomers possible for hex-3-ene-2,5-diol, CH3CH(OH)CH=CHCH(OH)CH3, is

The molecule has a double bond (E/Z) and two identical stereocentres (C2 and C5), so it is symmetric. For each geometry the centre configurations are $(R,R)$, $(S,S)$ and $(R,S)$; the last is a meso compound because the two halves are mirror related, and $(S,R)$ is identical to it. So each geometry gives $3$ stereoisomers. Total $=2\times3=6$ (not $2^3=8$, which ignores the symmetry).