There is no reasons why you can't have one, it's just that manufacturers identify their market and how much it costs to produce a car they feel they can sell. Theres no point spending millions making a car which people will not buy. There are different types of vehicle for which there are requirements as to weight, construction, use standards, testing etc. They are the same for all vehicles up to 8 passenger seats. This is called M! category. It then goes into M2 which is any passenger carrying vehicle that can seat more than more than 8 passengers, excluding the driver, up to a laden mass of 5 tonnes. Category M3 is any passenger carrying vehicle that can seat more than 8 passengers, exluding the driver, having a laden mass of more than 5 tonnes. Taking all this into account, the best thing to make is a 7 passenger seater