You're right Rick, Scalextric Digital has a maximum of 6 cars. The same applies to Carerra D132, if I'm right Carrera D124 has a maximum of 4 cars.
Both brands are made by "toy" factories, who aim a the home market.
Sytems like Scorpius and Oxigen are aimed at the clubs, a completely different ball game!
The question should be if 6 cars is enough for home racing with friends and family?
The average family size in the UK is 2.36, other west European countries are also around 2. So an average family would be about 4 persons, and I don't count single parent households.
Per car you need a certain amount of track, let's say 3 meters. This means that 6 cars will need 18 meters, how many families own a house big enough for such a track (and the space for the 6 drivers)? I think it's a minority.
Imho SSD and D132/D124 are perfectly suited for home racing, I see no need to build a digital track at home for 15 to 20 drivers.
Hence you should not compare apples with pears