Two. One to take out the bulb and drop it, and the other to try and sell it before it crashes (knowing that it's already burned out).
or
Because all his other wives support Hillary.
or
Two. One to screw it in, and one to stab the other in the back and take all the credit.
or
1,622. One to tell the original joke, and the rest to submit give some minor variation of it! For example:
or
Sunday!
or