The redox couple

A redox reaction is always built up of two half reactions:

The equation of couple 1: red 1 ox 1 + electrons
The equation of couple 2: ox 2 + electrons red 2
Total equation: red 1 + ox 2 ox 1 + red 2

The number of electrons involved must satisfy the rule:

ALWAYS THE SAME NUMBER OF ELECTRONS IS DONATED AS IS CAPTURED

Electrons do not get lost, are not just created from nothing, and so:
You must always confirm and certify the two half reactions in that way: the number of donated electrons must equal the number of captured electrons.

Example:
The equation of couple 1: Al Al3+ + 3e- | x 2
The equation of couple 2: I2 + 2e- 2I- | x 3
Total equaition: 2Al + 3I2 2Al3+ + 6I-

The total equation is only reliable if the number of electrons in half reaction 1 equals the number of electrons in half reaction 2.
In case of insolubility of the salt Aluminium Iodide, there will be a secondary reaction: the precipitation of an insoluble salt.
Att.: This is not the case in the example; check that in table XI: solubility of salts in water.