Only one, but she/he'll have to go out and buy the light bulb adaptor card first, which is extra.
or
Your wife comes back to you, your dog returns to life and you get out of prison.
or
Because it might crack up!
or
Seven. One to change it and the other six to sing about how good the old one was.
or
Just one, provided there is a programmer around to explain how to do it.
or