Think I've found the answer on confused.com website
With two cars, a driver will need two lots of NCB - one lot for each car. Drivers can't share a NCB earned on one car between two cars.
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.