How many properties are maintained by the amusement park corporation that first screened the film The Sensorium in 1984?
The Sensorium: The Sensorium is regarded the world's first commercial 4D film and was first screened in a Six Flags theme park in Baltimore in 1984. Six Flags:  The company maintains twenty properties throughout North America including theme parks, thrill parks, water parks, and family entertainment centers.
twenty