As stated in the article, they need to reach 11 billion miles to get the confidence level they are looking for. So extrapolating this, it would take 5500 cars a decade of round the clock testing to actually meet the guarantees they need (90% safer than human driven at a 80% confidence level). You still could do it in parallel; it'd just require a lot of test cars.

One part that is unclear is that the 11 billion mile requirement doesn't specify urban conditions and the 2 million mile per decade estimate does. So there might be some errors there.