E is an intensive property because the voltage difference is the same and does not depend on how many times the reaction occurs. As a result, E will not change even when you multiply the reaction by a coefficient.
When the chemical reaction is multiplied by a factor, delta G and delta°G is increased by that factor, but Ecell and E°cell stays the same. This is also because cell potential is independent of the cell size.
It simply comes down to E being an intensive property, meaning that the difference in voltage will not change if the reaction takes place more than once whereas gibbs free energy, for instance, will change.