This is not a philosophical discussion, but I figured if there was anyone to get this joke (or probably to have heard it before) it would be people on this thread.
(Please note this joke is not meant as an insult to existentialists or anyone else.)
How many existentialists does it take to change a lightbulb?
Two. One to change the lightbulb and one to observe how the lightbulb symbolizes an incandescent beacon of subjectivity in a netherworld of Cosmic Nothingness.