The NES has 17 as per the list from Wikipedia. Not sure which system has more.

The Adventures of Bayou Billy (gun optional)
Baby Boomer (unlicensed) (gun optional)
Barker Bill's Trick Shooting
Chiller (unlicensed) (gun optional)
Duck Hunt
Freedom Force
Gotcha! The Sport!
Gumshoe
Hogan's Alley
Laser Invasion
The Lone Ranger (gun optional)
Mechanized Attack
Operation Wolf (gun optional)
Shooting Range (requires gun and controller)
To The Earth
Track & Field II (Gun compatible)
Wild Gunman