Nobody knows. Anybody that cites numbers is pulling them from a very biased sample. With the exception of a few countries, most of the testing is done on a very self-selected set of people. Those with symptoms who were ill enough to get tested and willing deal with a with a 14 day quarantine if they turned up positive.

Even counties with good, pervasive testing can’t test for how many people already had it and are carrying antibodies.

We are flying very blind here. Treat all numbers with a huge grain of salt.

It's by definition. If you can see 3 ducks in your house, that means there are 3 or more ducks in your house including the ones you didn't see.