When it comes to how teeth are prepared for veneers, it can vary greatly from patient to patient, based on several parameters. In any case, it should be the dentist’s goal to remove the least amount of natural tooth structure possible to get the desired shape for the final result.
If a patient has virtually no crowding and slightly undersized teeth, the veneers can be sometimes made with no preparation to the teeth at all. This is not often the case, but as a general rule otherwise, it is most ideal to keep the preparation within the enamel (the outer layer of the tooth). This is desired because not only is it more conservative, but also bonding a veneer to enamel rather than dentin produces a superior adhesion to the tooth.
There are some cases when there is a significant amount of crowding, misalignment, or existing fillings/restorations and further preparation into the dentin is necessary to get a satisfactory result. This can still be predictably addressed, but more tooth structure must be removed. Typical preparation in this example would involve removing1-2mm from the biting edge, front, and adjacent sides of the teeth.