So use a 0.5% IFR and 8 day time to double implies 12 million in the US have/had it and there are 40 times more cases than we've actually detected. It's plausible, but seems high.

Those numbers double with a 6 day time to double (24 million infected, 80 times more than we've detected).

Assume a fatality rate of 0.2% (twice as bad as flu) and a 6 day time to double and you have 60 million cases.

One more just for fun: Let's assume an IFR equal to the flu and 6 day time to double. Now 120 million have/had it. 1/3 of America.