Four. One to change it, one to sing about how heartbroken he is at the loss of the old one, one to sing about how madly in love he is with the new one, and one to go "Yee-Hah!" and throw his cowboy hat in the air.
That depends on the speed of the change and the mass of the bulb. Or vice versa, of course. It just might be easier to leave the bulb and change the room. It's all relative.