How much hair you need depends on how many tracks you have. For example: If I had hair to my chin and I wanted hair to my bra line (about 18" long) I might need 5 or 6 tracks/wefts depending upon my current haircut. If I had hair to my shoulders that was thin and stringy and I wanted it thicker and more luscious but the same length I might need 2 or 3 (or more) tracks/wefts.
Once the tracks are done, thehair wefts are sewn to them and then your own crown area hair is blended with and combed over the extensions.