Two — one to change the bulb, and one to write a song about how good the old light bulb was.
or
The barman says "I'll serve you, but don't start anything"
or
Cause your the only one Hair.
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Only one, but she/he'll have to go out and buy the light bulb adaptor card first, which is extra.
or
Seven. One to change it and the other six to sing about how good the old one was.
or