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 (ACS), a rolling survey designed to replace the decennial census Long Form, offer a wide variety of tabulations based on the questions asked. See the tabs above for more information about the data compiled in the decennial censuses and, more recently, in the American Community Survey.
For additional sources of statistical information, see: Statistics Sources for the Social Sciences