In this case four: 3 to build the scaffolding and 1 to plug the bulb in:
I know they are only doing their job (at my request) and following the rules, BUT they are all wearing safety boots, hard hats, industrial gloves and are carrying goggles (and no doubt hearing protection). This is my house, and they are CHANGING A LIGHT BULB. So far it has taken 30 minutes to build the scaffold ...
(mumble mumble rhubarb rhubarb mumble mumble)