HMRC does not stipulate numbers when asessing whether somebody is actually running a business. Their rules state that anyone who "carries out a transaction or a series of transactions with a view to making a profit" is defined as carrying on a trade.
In reality, most people who only own one car at a time and change that car frequently, will generally not be looked on as a car trader.
However, if you are going about buying batches of cars, holding them in stock, advertising the car sales regularly etc, then it could be held that you are indeed in business.
The problem is that it is up to the individual to notify HMRC that they have commenced trading and there are fines and penalties for failure to notify. There are also fines, penalties and ultimately prison sentences for those who fail to complete self assessment tax returns when they should and notifying HMRC of the profits thay are making and paying the Income Tax and Class 4 National Insurance arising on those profits.