Number of stereoisomers and symmetry of pentane-2,3,4-triol
It is easier to compare molecules is you rotate them all into a comparable position.  Say 1 with the central C-3 hydroxyl group out of the plane of the screen/page like in 1, 4, 5 and 7.

So when we rotate a molecule by 180 degrees along a vertical axis in the plane of the paper, something going into the plane of the page on one end off the molecule rotates to become something coming out of the plane of the page on the other end of the molecule.  Also central group flips from into to out of the page and vice-versa.



Hence we can tell that actually pairs (1,2), (3,4), (5,8) and (6,7) are the same molecule, just rotated by 180 degrees.

The absolute positions of the three centres doesn't matter, only the relative orientation of the end two to the central site.