Unfortunately, some people don't understand what exponential growth is. The proliferation of bacteria, infections with a virus and many processes in nature happen with exponential growth. In the case of Corona, it is determined by the factor that one infected person usually infects 3 others.

European countries have observed that every three days the number of infected people is doubling. The growth is slow in the beginning, and from a certain point on it explodes.

If one is optimistic and supposes that numbers only double once a week in the US AND forget about the massive dark figure, you'd end up here:

March 16: 4,000
March 23: 8,000
March 30: 16,000
April 07: 32,000
April 14: 64,000
April 21: 128,000
April 28: 256,000
May 05: 512,000
May 12: 1 million
May 19: 2m
May 26: 4m
June 2: 8m
June 9: 16m
June 16: 32m
June 23: 64m
June 30: 128m
July 7: 256m.

That was a very optimistic scenario for the case that no restrictions are put in place.