As of mid-2018, there were about 327 million people in the U.S., compared with 138.5 million housing units.
With almost 120 million households in the U.S., there were then about 18 million vacant housing units. These are either between occupants - for sale or rent, or rented or sold but awaiting new residents - or second homes used occasionally.
The 2010 Census counted around 130 million housing units in the United States. Of these housing units, nearly 115 million were occupied.