But the majority do have 4 exhaust valves per cyl....... there are some 2 valve units out there -71 series that are prolly older than most who post here
My first Scout boat had a single 2-valve 6-71 main engine. If I recall correctly, the boat was originally powered by an inline-8 Caterpillar, but a ferry boat company had bought the boat from the BOP and pulled the Kitty-Cat out to use in one of their other boats. So, my Scout unit got the boat (sometime between 1965 and 1970, so long before my time) without an engine, and supposedly found this engine in a WWII-era landing craft. The injectors were so fouled up that she was only running on 3 cylinders for the first couple of years

They were too cheap to install throttle controls in the wheelhouse, so they had an EOT and an engineer down there at all times. You can see the throttle handle and gearshift lever in the picture. THOSE were some fun times
This boat also had a (also 2-valve) 2-71 genset. We didn't have a tach on the generator, but she HAD to have been turning over 2500 RPM...we cruised at 1750 RPM on the main engine and the genset STILL drowned out the main

I have no idea why it was set up to run so fast; I've been around other 2-71 gensets that run at a much lower RPM and are MUCH quieter.
I drove them crown buses for many years, both five and ten speed. I have to say, if someone has never been around one or driven one, they are really missing out.
I spent heaven knows how much time riding in them as a kid; I always thought they were cool because of the sound, the body shape, and the manual transmission. I was reading up about them, and apparently Crown buses aren't very common east of the Rockies, and they're built like tanks! I've always wondered what it's like to drive one

someday, when I have time, disposable income, and space...