The White Walkers take
Craster's sons and turn them into more White Walkers.
There was a scene where a White Walker
converts a baby into another White Walker,
surrounded by other White Walkers. There should be an army of White Walkers by now, not considering the wights. But when fighting with the
Wildlings/Night’s Watch/Brotherhood Without Banners/random tag-alongs and the dragons
the destruction of The Wall
too few White Walkers were shown. Where are the rest of them?