Since the early days of religion in North America, denominational and missionary groups have collected data on their membership and activities. Throughout the 20th and 21st century, the sociological study of religion has led to further and more nuanced research into religious beliefs, customs, and communities in the United States. The ARDA has compiled much of this research, but denominationally-specific data can more likely be found on a denomination's own website (or archives).
For instance, the ELCA gathers data at the congregational, synodical, regional, and national level. The Lutheran World Federation compiles national statistics, as does the World Council of Churches. If you're having difficulty finding information about a particular denomination, contact the librarian for more help.