One per house, like everyone else!

Plebeian.

One per floor per house, of course. You don't want to have to carry them up and down stairs.

Nouveau riche.

All houses should have one floor. Why use stairs or elevators when you can just buy more land?

The staff are downstairs, and they have their own cleaning equipment.

Not the same person, but I have three. I have a roomba for everyday sweeping, a Bissell for actually cleaning the carpets not just sweeping them, and a Black and Decker handheld for the stairs, the car, and other places it's hard to take a full vacuum or a robot.