Estimates have said that they expect 20%-60% of people to get it (originally [1] but updated according to [2]. Something like 10-15% of those people require hospitalization [2] and something like 0.5% - 4% of people die [4]. If we use the lower bounds on all those we are talking about 6.5 million Americans hospitalized and 327,000 dead. For the sake of comparison, that is almost half the number of Americans who have ever died of HIV/AIDS [5].
[1] - https://www.cbsnews.com/news/coronavirus-infection-outbreak-...
[2] - https://twitter.com/mlipsitch/status/1234879949946814464?s=2...
[3] - https://www.statnews.com/2020/03/10/simple-math-alarming-ans...
[4] - https://www.worldometers.info/coronavirus/coronavirus-death-...
[5] - https://www.kff.org/hivaids/fact-sheet/the-hivaids-epidemic-...
EDIT: I originally didn't post sources, so I backfilled those in and slightly tweaked the numbers to match.
EDIT 2: I adjusted the numbers again because as a reply points out one of my sources updated their estimates.