if it is a drawn/painted 3-D Object, it is only
one.
Just each person describes the side he was looking at, but not the entire object.
if this isn't a valid answer, it is not to find in nature and just a painting, like some/many of Escher's objects.
Reasoning; The first topmost part of the "flat" object from left to right is sharing it's upper parts lower edge with the middle part, which the observer on the right is seeing.
https://www.lesswrong.com/posts/bcM5ft8jvsffsZZ4Y/the-parable-of-hemlock
