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.

Based on estimates that I have heard, the cost of a single car can be upwards of $300K. This limits putting large fleets of cars on the road only to the well funded (Google/Waymo, GM, Ford, Uber, etc) for only testing purposes. Smaller upstarts would need those cars to start taking rides and generating revenue a lot sooner. This seems like a worthy innovation to bring down costs and encourage more competition in the space.