How many electrons are involved in the following redox reaction? Cr_2 O_7^2-+ Fe^2++ C_2 O_4^2- → Cr^3++ Fe^3++ CO_2 (Unbalanced)

How many electrons are involved in the following redox reaction? $$\begin{aligned} & \mathrm{Cr}_2 \mathrm{O}_7^{2-}+\mathrm{Fe}^{2+}+\mathrm{C}_2 \mathrm{O}_4^{2-} \\ & \quad \rightarrow \mathrm{Cr}^{3+}+\mathrm{Fe}^{3+}+\mathrm{CO}_2 \text { (Unbalanced) } \end{aligned} $$