There is no common measurement for how much yardage is in a ball or skein of yarn.
You'll need a little over five to get the necessary yardage, so buy six and you should be fine.
If I had a pattern for a scarf similar to the length I was looking for and around the same gauge as the yarn I was looking at, I might assume that the yardage requirements would be similar.
Penguin or Pingu launch the rules are simple you simply batter the poor defenseless creature with a club and attempt to gain maximum yardage.
Nail down the stroke technique first, then think about adding the yardage.