Here's one source: https://www.washingtonpost.com/health/2020/02/25/cdc-coronav...

> While South Korea has run more than 35,000 coronavirus tests, the United States has tested only 426 people, not including people who returned on evacuation flights.

> not including people who returned on evacuation flights.

This bit seems important. The “only 400 tested!” statistic people are citing seems much more reasonable in this context.

If you assume that only 400 other people in all of the USA have been travelling beyond its borders and/or in contact with such travellers, then it's ok to assume that all potentially infected have been tested.

The US placed tight restrictions on people who'd recently visited China entering the country. Other countries did not. Testing regimes elsewhere have been focused on visitors from China and people who've been in direct contact with them.

Here you go: https://www.cdc.gov/coronavirus/2019-ncov/cases-in-us.html