That 'original estimate' is from the paper that 'alexandercrohde linked, and that's pretty much exactly what his original estimate said.
> In the (unlikely) absence of any control measures or spontaneous changes in individual behaviour, we would expect a peak in mortality (daily deaths) to occur after approximately 3 months(Figure 1A). In such scenarios, given an estimated R0 of 2.4, we predict 81% ofthe GB and US populations would be infected over the course of the epidemic. Epidemic timings are approximate given the limitations of surveillance data in both countries: The epidemic is predicted to be broader in the US than in GB and to peak slightly later. This is due to the larger geographic scale of the US, resulting in more distinct localised epidemics across states (Figure 1B)than seen across GB. The higher peak in mortality in GB is due to the smaller size of the country and its older population compared with the US. In total, in an unmitigated epidemic, we would predict approximately 510,000 deaths in GB and 2.2 million in the US, not accounting for the potential negative effects of health systems being overwhelmed on mortality.
"In the (unlikely) absence of any control measures or spontaneous changes in individual behaviour" means "assuming people do nothing."
Can you please provide a link to more info about the 20k number?
EDIT: thanks to 'alexandercrohde, the 20k number is in fact for the UK, not the US, meaning that it is from this same paper too (see rest of thread)