Actually we don't.
We have models of what may or may not be. As Dr. Fauci said in a statement, "these models are only as good as the variables we put in". If they are anything like the climate change models for the past 40 years, they will be extremely overstated.
Until we have better testing and a better understanding of mortality across the spectrum of those infected, everything is a variable. Right now, they think about 80% of the cases are mild and unreported. We only know about serious cases so far. We don't know how many are or have been infected and what the true recovery rate is. So any numbers being put out now are a wild ass guess based only on reported cases that require medical intervention.
It is serious, but the best advice is Don't Panic and take precautions that are correct for you.