Two: the big lithium-ion traction battery, and the small 12v accessory battery. The purpose of the 12V battery is to power various computers to "start" the car, and to power various electronics when the system is off, such as the keyless entry; that is, it does not have to start the gasoline engine.
It is similar to ALL hybrids, only the main traction battery is bigger and the computer program for USING the battery power is somewhat different.