Why the Number Isn't Fixed
A veneer's job is to change how a specific tooth looks, so the number needed is really a question of how many teeth need changing, not a standard package. Someone with one discolored or chipped front tooth might only need a single veneer, or two if matching it evenly requires treating the tooth next to it as well, since shade matching a lone new veneer against older teeth can be trickier than it sounds. Someone unhappy with the overall shape, color, or evenness of their whole smile is a different case entirely. Both are legitimate starting points, and neither one is automatically the correct amount of treatment. The goal is matching the plan to what's actually bothering you. Talking through specific goals before counting teeth is usually a far more productive way to plan a case than starting with a target number and working backward from it.