Think I've found the answer on confused.com website
Been having a debate about this with my Mum, she seems to think that you can effectively have 20 cars all with X no claims bonus all insured to the same person, so for example you could have a metro for 10 years, then win the lottery and buy 19 other cars and have the same number of no claims on each car.
your mum is wrong unfortunately but there are ways around it.
1) speeking to the insurance company for the second car they may well give you a few years ncb to start with on that policy anyway.
2) multi-car policies though god knows if theres a limi to the number of cars on the one policy.