How many collisions are there in a car crash?

That would depend on how many vehicles are involved. If only two vehicles are involved, and they both stop after the crash, then there is one collision

That's not what is meant. There is ALWAYS more than one collision when a car crashes. There are THREE (3) collisions in every crash. The car itself hitting the other car; your body hitting the dash or other part of the car depending on where you're sitting; and finally the collision that is inside the body, your organs and bones hitting against the inside of your body.