Since primer provides a rough surface for paint adhesion compared to a smooth glossy finish , you need primer to ensure that the paint will adhere well. If primer is not applied at all, this may cause the paint's finish to come off easily because primer provides a good foundation for adhesion.
Deciding how many coats of primer a vehicle needs depends on the vehicle. If the primer covers the vehicle without any problem and there is no body work to do, only one primer coat is needed to paint it.
The primer will act as a filler by filling in any blemishes or depressions where you can later apply spray paint without problems. Applying primer over bare metal is always beneficial but if there are no problematic areas then applying one coat of primer should be sufficient before painting your car.
Another variable is how strong or bold the previous colour is, this may make it necessary to apply more than one coat of primer. However, it is not necessary to over apply primer with so many coats. As long as the primer applies uniformly over the previous colour, then one or two coats should be sufficient.
You should also sand the primer down to a smooth finish before applying primer over primer as this will affect adhesion.