The accepted custom is to have five sets of double knots, which leaves four sections that are filled in with the wound thread. The Ashkenazi custom is that the first section has seven "winds", the second eight, the third eleven, and the fourth thirteen (Magen Avraham)
Why seven, eight, eleven and thirteen? The number seven represents the perfection of the physical world (the seven days of Creation, seven days of the week, etc.). The number eight represents the realm that is beyond the physical. The seven and the eight added together equals fifteen, which is the numerical value of the first two letters in God’s Name (Yud and Hay).
The number eleven equals the numerical value of eleven, the last two letters of God's Name (Vav and Hay). Together these three sections equal the numerical value of the Four Lettered Name of God.
The word "Echad", one - aleph (1), chet (8), dalet (4), equals the final section of thirteen. The number thirteen also represents The Thirteen Attributes of Mercy.