Amount of enantiomers
The compound in question is called D-galactose in carbohydrate chemistry. It has 4 chiral centers (C2, C3, C4, &amp; C5): Thus, stereochemistry of them are $(2R,3S,4S,5R)$. The rule is if a compound has $n$ chiral centers, then it has maximum total of $2^n$ possible stereoisomers, if it does not have any symmetry. D-galactose does not have any symmetry within the molecule (e.g., It would have symmetry if aldehyde group, $\ce{-CHO}$ has been reduced to $\ce{-CH2OH}$). Thus, four chiral centers of D-galactose are accounted for a total of 16 stereoisomers ($2^4$). If you draw them all, you would find they are collection of 8 pairs of enantiomers as:

{$(2R,3S,4S,5R)$, $(2S,3R,4R,5S)$}
{$(2R,3R,4S,5R)$, $(2S,3S,4R,5S)$}
{$(2R,3S,4R,5R)$, $(2S,3R,4S,5S)$}
{$(2R,3S,4S,5S)$, $(2S,3R,4R,5R)$}
{$(2S,3S,4S,5R)$, $(2R,3R,4R,5S)$}
{$(2R,3R,4R,5R)$, $(2S,3S,4S,5S)$}
{$(2S,3R,4S,5R)$, $(2R,3S,4R,5S)$}
{$(2R,3R,4S,5S)$, $(2S,3S,4R,5R)$}.

That make maximum total of 8 pairs of enantiomers, meaning $2^3 = 2^{4-1} = 2^{n-1}$, where $n$ is number of chiral centers. Therefore, your formula should have been for pair of enanthiomers.