My own personal experience tells me that you need 6.
1 to change the light bulb.
1 to hold the chair which the first is using to reach the socket.
1 to give the bulb to the one in the chair because he forgot to bring it before he climbed.
1 to turn the switch on/off to be sure the bulb is working properly.
1 to tell the rest that everything is going ok.
1 to alert the bulb changer to hurry up because the sensei is coming
In our dojo we followed the procedure twice a session.