That's why he delegates, I guess.

For a very long time, people believed that not only was Hell real, there are trillions and trillions of devils and their assistants running all over the place, tempting people, ruining crops, souring milk and so on. And they were all invisible. =P
It's a frightening mental picture, but it raises some important questions about HR and payroll in Hell, who keeps track of all these devils and imps, and how do they manufacture enough One Rings to keep everyone invisible.