The formula of an oxo-anion of manganese $\left(\mathrm{M}\mathit{n}\right)$ in which it shows the oxidation state equal to its group number (7) is ${\mathrm{MnO}}_4^{-}$.
$\mathrm{Mn}+\left(-2\times 4\right)=-1$
$\mathrm{Mn}-8=-1$
$\mathrm{Mn}=7$
Oxidation state of $\mathrm{Mn}$ in ${\mathrm{MnO}}_4^{-}$is + 7. The group number of $\mathrm{Mn}$ is also 7.