The number of electrons involved in the conversion of $Mn{O}_4^{-}$ to $Mn{O}_2$ is ____

Oxidation state changes from +7 to +4.

$\stackrel{+7}{\mathrm{Mn}}{\mathrm{O}}_4^{-}\to \stackrel{+4}{\mathrm{Mn}}{\mathrm{O}}_2$

Change in electrons is 3 (7-4=3), therefore the electrons involved in the given conversion is 3.