A faucet is fundamentally a geometry problem that has to be solved before the vanity top is fabricated and templated. The first major decision is the hole configuration — how many holes are in the countertop, how far apart they are, and what size they are. That configuration is already irreversibly fixed once the countertop is drilled and templated. A single-hole faucet needs one 1.25-inch hole drilled in the center. A three-hole faucet needs three separate holes, usually spaced 4 inches apart for compact layouts or 8 inches apart for spreads. A widespread configuration has the handles standing far apart from the spout — typically 8 to 16 inches apart — which changes both the visual aesthetics and the spatial demands on the vanity. Each hole configuration demands different things from the surface it mounts to, and fundamentally changes what faucet styles can actually fit in the space you have.
The second major decision is mounting type: deck-mount or wall-mount. Deck-mount sits directly on the countertop surface, with supply lines and waste connections coming up through the countertop or running down behind the vanity cabinet. Wall-mount anchors to the wall behind the sink and projects out from the wall, requiring the wall behind to be structurally solid, properly framed, and positioned close enough that supply lines can reach it. Wall-mount also irreversibly commits the wall to a specific height that cannot be changed later without replumbing. Wall-mounted faucets can give a vanity top the look of a clean, continuous surface with no visible faucet holes, but they demand more preparation of what is behind them and less flexibility for future changes.
The spout projection and reach have to be carefully matched to actually reach over the bowl without splashing. A spout with too short a reach barely clears the bowl rim and leaves water splashing down the front of the vanity top. Too much reach and the spout is awkward to use, projects too far from the wall, and looks visually wrong in proportion to the rest of the fixture. The reach is measured from the mounting surface to the tip of the spout when water flows. The actual bowl itself — the specific model that was chosen, not some generic dimension or average — sets the precise reach requirement. If the bowl changes after the faucet is chosen, or the faucet is being replaced, the reach must be re-evaluated against the actual bowl.