It's complicated.
The short answer is that not all aspects of engine performance scale linearly. That is to say that doubling the number of cylinders doesn't necessarily double some other parameter- it may triple it or some other ratio. Of course this also apply to negative parameters as well- more cylinders can have a negative or positive influence, so more is not necessarily better, depending on your design goal..
My guess is that cylinder count in a car engine is usually determined by a couple of chief factors- cost, overall engine size, weight, balancing forces and torque/power curve desired.