The 2009 American Housing Survey indicates that 30 percent of all housing units in the United States are renter-occupied or vacant for-rent, and a startlingly large share of total rental housing stock consists of single-family detached homes: 27 percent, according to the National Association of Home Builders’ blog. 
When adding townhouses (single-family attached units), the share of the rental housing stock due to single-family homes is roughly one-third (34 percent rounded). A large share is also due to homes that have been divided into two to four apartments with some common space, which represent 20 percent of the rental housing stock
Subtracting out manufactured housing units, this leaves only 44 percent of rental housing due to traditional multifamily apartment buildings of five or more units.
For more information: http://eyeonhousing.wordpress.com/2011/07/20/the-rental-housing-stock/