A lookup follows one path from the root through intermediate levels to a leaf. The number of nodes on that path depends on the tree’s height, not directly on the table’s row count. Each index page can contain many key-and-pointer entries, allowing a large table to be searched through a shallow tree.
Editor’s note: Examples such as “about three pages for one million rows” are illustrations rather than guarantees. The actual height and I/O depend on key size, page occupancy, row size and fill factor; measure the real execution plan.