Doesn't matter how many keys they give you, nobody really verifies or records it when you get new ones - lost my key need a spare, OK guvner, you don't even need a V5 to do it. I think it is Mercedes that cracked down a lot, now the only place for a key to a new Merc is Mercedes - think it's Mercedes - but I know one manufacturer has stopped it's keys being done by outsiders, the key is produced who knows where and shipped to the dealer, the key can only be coded by the system that is connected to the Factory and has to be done within a set time from the keys issue, the coding process is initiated by the installer but from there it is 100% hidden even from the installer. This would help a lot because a record of how many keys the owner has had would be readily available.
there are scammers out there that sell them to you then nick them back
Yup classic especially with newer cars bought secondhand privately. Buy a car, get set of keys made, sell car with original keys and docs - make note of owners location when completing V5C - wait a couple of days and take it back, drive car to a lockup someplace and wait a while, even forensics struggle because it 'was' their car. It will be parted out and on E-Bay over the next few months.
Any car sale verify how long they have had the car, make sure that the address you collect from is the one on the V5, no V5 no sale - sorry I lost it - yeh right sure you did go get a new one then call me.
Only defense against this is getting the key transponders changed out and the cars immobiliser reprogrammed which is a very expensive business.