30M population, 28M citizens, 20M registered, 3.8M not registered, 5M no response to census
Source, 2016 census data:
https://www.census.gov/data/tables/time-series/demo/voting-and-registration/p20-580.html
And all other kinds of breakdowns as well, including 2018. I went to 2016 because it was a presidential election year