How to start subequations with a known number?

Not sure why you would do it, but here it is.

documentclass{article}
usepackage{amsmath}

newcounter{storesubequations}

begin{document}

begin{equation}
y
end{equation}

begin{subequations}
begin{align}
a&=b 
c&=d
end{align}%
setcounter{storesubequations}{value{equation}}%
end{subequations}

addtocounter{equation}{-1}
begin{subequations}setcounter{equation}{value{storesubequations}}
begin{align}
e&=f 
g&=h
end{align}
end{subequations}

begin{equation}
y
end{equation}

end{document}