>>28460407
You've gotta fit all those valves in the same circle, with enough space between them for the head to not crack from being too thin. The more valves you have, the smaller they have to be, and eventually you'll actually have less area for air to travel through than if you had fewer, larger valves. Plus the extra complication in the valvetrain costs a lot more for diminishing returns, which is hard to justify for mass produced engines.