Touching 12 volt battery terminals is indeed very, very unlikely to harm you.

However, cars are equipped with ignition coils to produce a spark for the spark plugs. When a car is running the ignition coil is producing output of 20-30 kV and back EMF in the hundreds of volts. Normally this is filtered out from the 12-volt side in order to protect the electronics and battery, but I would not recommend trusting those filters with your life.