The mortality rate is currently just a guess, and is hampered by the time lag between first symptoms and outcome (recovery or death) being weeks, and lots of new cases being reported in that time.
20% of cases are severe enough to require oxygen etc. As someone else stated upthread the outcomes of the cases at their conclusion, is currently 82% recovered 18% dead.
During SARS the WHO also estimated the mortality rate to be 2%. Afterwards when all the figures were collated, it was confirmed as 9.6%. This virus is 97% genetically the same as SARS.
These are all facts from sound scientific sources. The governments, and WHO etc, aren't wrong that if this is allowed to spread (and it's highly infectious), it could be extremely serious.
Quarantine, now just as it was in the Black Death, is an effective way to prevent spread. One village in Germany during that time closed their gates and set up a rotation of guards and didn't have one case. Obviously our lives are very much different to then, but quarantine is still our best tool until a vaccine or suitable antivirals are found. Of course hygiene as well.