According to the datasheet, this part can withstand ±12kV Human Body Model (HBM) ESD on CANH and CANL. Is this assuming any other external protection?
The device can withstand ±12kV HBM ESD without any added external protection.
Is that a one shot rating?
No, it is not one shot. In Maxim's standard ESD testing methods we apply 10 shots.
Would this part be expected to survive if we apply ±8kV 10 times (5 positive, 5 negative) in an HBM ESD test without any other external ESD protection?
Yes, the part should be able to survive the ±8kV 10 shot HBM test on both positive and negative side without any added external ESD protection.