It would be +7 oxidation number.
[All compounds are zero]
Al is in group III [ or 13 depending on the system ] so the oxidation state is 3.
You have 12 O at -2 each for-24. That leaves you at -24+3=-21. So, 3 Cl atoms must be +21 to balance the -21 there which makes +21/3 or +7 for each Cl.
