It depends on the look you are trying to accomplish and the amount of volume you like for your hair. For a full head, we recommend two bundles for lengths under 18 inches and three to four bundles for lengths above 18" to achieve desired fullness. The longer the length, the more hair you will need as each bundle is approximately 100g, regardless of the length.
If you will be finishing with a closure or a frontal, you may need ness hair to finish, as you will be using less hair from your last bundles.
If you have any other questions about how many bundles you need for your hairstyle, feel free to comment below!