n is the no. of electrons in anode or cathode reaction.

If no. of electrons are different then make these no. common

for ex. if reactions are 

             Al ---------------> Al⁺⁺⁺ + 3e^-

and     Cu⁺⁺ + 2e^- ------------> Cu

then multiply (1) to 2 and (2) to 3

after that electrons will be 6 in both reactions so n = 6