I use either the knot method, or a home made reroot tool... Currently I use the knot method, since all my needles are broken.
Aaaand about the amount, it depends on how heavy you root. I like my hair to lay a little thinner, and one hank of Barbie hair is enough for me to reroot an MH girl. But some people find that even two isn't enough for them.
So it's a preference thing.