Each decoy launcher is a box launcher and fires once without reloading at a base or in a hanger. So to have 8 decoys you need 8 decoy launchers
Of course you have to build decoy missiles for the same reason you build normal missiles they are missiles and have an economic cost
If a ship has no decoy launchers it has no decoys, you may have put some decoy missiles in the magazine but they are decorative and do nothing. If you have not added decoy launchers or put pointless decoys in the magazine there are no decoys onboard. If you think there are post the screen shot which is confusing you and someone can explain what is wrong.
Decoys do not decoy 1 missile, they decoy a proportion of the incoming missile wave, if that is 1 missile you probably wasted them. They are intended to help survive when someone launches a wave of 300 missiles from box launchers at you.
So your interpretation is wrong as if it were correct they would be pointless
I suggest building some ships with decoys, then building some other ships with missile launchers and firing missiles at the ships with decoys to see how it works