Pattern recommends 20 balls of Amy Butler Belle Organic DK.

From the book (page 71):-

Needles
1 Pair 4mm (no 8) (US 6) needles.
1 Pair double-pointed 4mm (no 8) (US 6) needles

Finished Size
Completed scarf measures approx 244cm (96 inches) and approx 16cm (6.25 ins) wide.