Lots of scenarios need to be covered, which is why Waymo has driven millions of miles in its vehicles so far. I don’t have a great sense of how many tests are in Waymo’s regression test suite, but I would guess into the thousands. Maybe tens of thousands. Most of these are what engineers would call “interesting” scenarios. Those scenarios are each vanishingly rare, but they’re what make the difference between putting the car on the rode without a safety driver, or not.