A note about population counts
The population is "counted" every 10 years via the decennial census. For population counts (total number of persons) between decennial censuses, see: Population Estimates
The decennial censuses (conducted every ten years) and the American Community Survey offer a wide variety of tabulations based on the questions asked. See below for more information about the data compiled in the decennial censuses and, more recently, in the American Community Survey.
Can the answer be found in the US Census?
U.S. Census Bureau Catalog and Guide
Especially useful for historical census volumes
Selected Census Products, below: American Factfinder, Social Explorer, Geolytics, County and City Databooks and print sources.
Print and Online Compendia, e.g. Statistical Abstract of the United States
County and City Data Books
Decennial Census and the American Community Survey
Print Census Volumes in the Library
PDF sources of early census years
Other census reports and surveys