Two. One to change it and one to yell "Ta-daa!" when he's done.
Just one, but it takes a really long time, and the lightbulb has to want to change...
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One, but the light bulb has to want to change.
All of them actually. Two to hold the ladder and one to change the lightbulb.
So he could yell "Anna 1, Anna 2!"
Motorist: The light just turned yellow.
Phillip