How many dogs fit in a car?

That may well depend on the size of the dog(s) and the size of the car. One possibility is to transport them in a specially designed cage or pet transporter, which would sit in the boot/trunk of the car or just in the boot/trunk itself - provided it's a hatchback. If the latter is the most feasible then a dog guard or dog barrier, separating the dog from the rest of the car, may (also) be required.