I think that the 20% comes from the percentage of serious cases (It was 20% in past, now it's 12% https://www.worldometers.info/coronavirus/ https://www.worldom

I think that the 20% comes from the percentage of serious cases (It was 20% in past, now it's 12% https://www.worldometers.info/coronavirus/ https://www.worldometers.info/coronavirus/ ). Problem with serious cases are that they need medical attention, so if all of us get infected medical system will not be able to handle 20% of population, so they will have to be without it... That being said, not all of 20% needs ICU, and not all people will be infected at the same time (specially now since people are panicking). OTOH, if medical system will be DDOS'd, then also other people will die that need medical attention (so mortality for other illnesses will go up). So in theory it could be as bad as 20% if we would ignore the virus, let it spread to everyone as fast as possible and don't do anything. But that is not what is happening.