Wefts are attached to your head by sewing them to a tiny braid made out of your own hair. The track (known to hairdressers as a corn-row) runs horizontally around the back of your head. The tracks are done in the middle of your scalp underneath your hairline so you can't see them.
The crown area of your own hair covers the top track making the extensions seem invisible. The number and size of tracks matches the number and size of wefts. The wefts and tracks ideally should be made in different sizes in order to match exactly the way your hair would look if you had grown it that way yourself.
 

Depending upon 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, the wefts are sewn to them and then your own crown area hair is blended with and combed over the extensions. From now on you have to think of the extensions→clips in hair extension as YOUR HAIR.