Touch ID/iPhone 8 Plus
I would suggest you, that you delete all the fingers that you have been scanned in for touch ID, because you may have registered some fingers several times.
Then scan in all the fingers that you want to use for touch ID and label them all by their names (left thumb, right middle finger...). Look here as example:

So you will have it under control which finger you have already registered and which not yet in order to avoid double registration!