Assertion A is true because $\Delta_r G$ depends on the number of moles of electrons transferred ($n$). Reason R is also true because $E_{cell}$ is an intensive property (independent of amount) and $\Delta_r G$ is an extensive property (depends on amount). However, R is not the direct explanation for why $\Delta_r G$ depends on $n$ in the equation, as the dependence on $n$ is a mathematical consequence of the definition of Gibbs free energy as an extensive property.