The United States has collected data about its population since 1790 and continues to collect data every 10 years. Currently, the National Archives has the 1790 to 1940 census records available to the public.
In addition to the censuses conducted by the federal government, many state censuses have been conducted. Some of these date to the colonial period. Although the National Archives may have some of these records, contact your state or local archives to inquire about their state census holdings.