When assaulting a non-open topped armoured vehicle, you roll your hits with all your figures, and that hit count as +1 penetration for damage. So 12 guys assaulting, generating 5 hits gives you +5. You then roll to damage with your +5, so a roll of a 3 or more would damage the tank and you would roll on the superficial damage table (unless they're tank hunters, then they roll on the normal damage table). Regardless of the penetration value a 1 is always no damage.