Passenger cars must **mandatorily have two brake lights**. These lights are bright red and are located at the rear of the vehicle. Their function is to indicate that the vehicle is braking, and they must be turned on whenever the driver brakes, even if it is for a short period of time, such as when stopping at a red traffic light.

There is a **third brake light**, which is also placed at the rear, but this is **optional** for cars. It lights up along with the other brake lights and has the same color, but it is not mandatory.

Therefore, the minimum number of brake lights that a passenger car must have is **two**.