Over 424,000 miles driven:
272 times the car had a 'system failure' and immediately returned control to the driver with only a couple seconds of warning. (Approx. every 1,558 miles.) A car mid-traffic spontaneously dropping control of the vehicle would likely create a large number of accidents.
13 car accidents prevented via human intervention (Approx. every 32,615 miles), 10 of which would've been the self-driving car's at-fault (Approx. every 42,400 miles). These virtual accidents were tested with the telemetry recorded during the incident, and it was determined had the human test driver not intervened, an accident would've occurred.
Total of these events is 285, which is approximately every 1,487 miles driven.
For useful comparison, a rough human average (when you add a large margin to account for unreported accidents) is somewhere around one accident every 150,000 miles driven. (Insurance companies see them every 250,000 miles approximately, I believe.)