Only one. Just as long as you hand him the damn lightbulb.
Apparently a lot, because that lightbulb still isn't screwed in.
Depends what you want it to change into...
A: Two. One to change it and one to sprinkle it with Parmesan.
We're not sure, they've yet to see the light.
A watchman.
Because if it were any longer it would be a foot!