Depending on the style your going for, the length you select, and the size of your head, a minimum of two bundles is needed for a natural-looking sew-in 14" or shorter. Three or more bundles for a full look. Longer lengths (18″+) typically require three bundles..
The number of bundles purchased depends on the look a client is going for. Some clients prefer three bundles over two bundles, some prefer four bundles over three. It varies by client and their desired style.