Among the following, number of compounds that would require electrolysis process to get their respective metal is ___________.$\mathrm{NaCl}, {\mathrm{Cr}}_2{\mathrm{O}}_3, {\mathrm{MgCl}}_2, {\mathrm{Al}}_2{\mathrm{O}}_3, {\mathrm{CaCl}}_2, {\mathrm{Fe}}_2{\mathrm{O}}_3$

Metals that lie at the top of the electrochemical series are very reactive and extracted by electrolysis process.

The electrolysis process is an extraction process in which an electric current is passed over a molten state of a compound and the ions get deposited on the electrodes. Oxidation and reduction take place at the anode and cathode, respectively.

Four compounds, that is, $\text{NaCl, MgC}{\text{l}}_{\text{2}}\text{, A}{\text{l}}_{\text{2}}{\text{O}}_{\text{3}}\text{, CaC}{\text{l}}_{\text{2}}$ require an electrolysis process to extract respective metals.