A sponge!
or
Five. One to screw it in and four to tell him how bitchin' he looked doing it.
or
Weeeeeeeeeeeeeeeeeeeeeeeeeee
or
Only one. But he'll leave a big puddle of spit on the floor underneath him.
or
Only one, but she/he'll have to go out and buy the light bulb adaptor card first, which is extra.
or