It depends on the layout and theme of your haunt. If you have actor access halls and other passages that easily and quickly allow an actor to move from room to room undetected by the customer, than one actor can cover several areas. A good example of this is Ghostly Manor; in the summer they usually only have a small handful of actors working the haunt, but you would swear there were dozens of them.
In regards to theme, make sure the actors that are covering multiple areas match all the rooms (ex. if it is several medical rooms someone in scrubs can easily cover it, however an actor in scrubs would look out of place in a cemetery scene or clown room).