According to NHTSA, it was about 2.3[0] accidents per million miles driven in 2016, or one accident per 0.43 million miles. However, you probably shouldn't directly compare those numbers without at least separating highway miles from surface streets, as Autopilot-enabled miles driven may have a very different profile than the national average in this regard (I'm not familiar with how Autopilot is used in practice).
[0] https://cdan.nhtsa.gov/tsftables/National%20Statistics.pdf ; figure obtained by dividing "POLICE-REPORTED MOTOR VEHICLE TRAFFIC CRASHES (Total)" by "Vehicle Miles Traveled (Millions)". See more fun stats at https://cdan.nhtsa.gov/tsftables/tsfar.htm