Originally posted by TwinTurbo_Supra:
V6- six cylinders, V position
V8- eight cylinders, V position
V10- ten cylinders, V position
W12- 12 cylinders, W position (eg. Continental GT, Bugatti Veyron)

for the question of an engine size, the whole deciding factor is the aims of the designer. A V12 has more cylinders, hence the gap timing between each of the cylinders' firing order is smaller, hence it is smoother. But there are so many factors to consider.
In terms of more cylinders, 12 vs 10 vs 8, in general more cylinders means more valve area, so u can in theory get more airflow in an engine per revolution. More cylinders also tend to have lighter pieces for a given capacity, so u r often able to run them to a higher RPM. And both of these things mean more airflow, to which you put more fuel, and that means you add more base power.
But what u do is u take off some things too. More cylinders give u greater losses thru friction. Though u are theoratically producing more power, because there are more moving parts so it takes more power for them to move, so in theory u get a situation where a 12 vs 10 vs 8, will each respectively have more power but a lot more losses, so that the actual bhp is different.
But there is obviously a lot more to it, because of the losses. More cylinders are less efficient, so you have to use much more fuel. In the end u've got a lot of things going against much more cylinders and only a few going for them.
Also considering the bore and stroke, in a V8, it has a large bore, which is god for generating power, and a short stroke, which is also good for boosting revs because the piston has less dist to travel, It has good torque. For a V12 though, the capacity must be much larger, because in any case, if u compare a V8 and a V12 both with the same capacity, the V12 is going to get better top end power because of the lighter pistons moving up n down, but it lacks low end torque.
Another problem is packaging. In designing a car, u have to decide how to package the engine. A V12, though more powerful, is longer and heavier, used more fuel, and its crankcase was so much longer that it is fundamentally less torsionally stiff. There are also more exhaust pipes to package. Also, operating with more friction, it is less heat efficient, which would imply that larger radiators are needed, which presented more packaging problems. This would also lead to increased drag, which could be undesirable for the want of a slightly superior top end power.