Why, according to Chapter 6 of All Quiet on the Western Front, do "between five and ten recruits fall to every old hand?"

In this part of Chapter 6, the narrator talks about ways
in which new recruits get killed because they do not know what they are doing.  He
mentions one reason right after the quote you provide us, and he provides further
reasons later in the chapter.

The first reason is because
the new recruits do not know when it is safe to take their gas masks off.  They don't
realize the gas settles into low places and that people in low places still need their
masks after people higher up are safe.

Other reasons are
implied a few pages later.  The narrator is talking there about all the things the new
recruits need to be taught.  We can infer that they die more than the old hands because
they don't know you can run from mortars, because they don't know how to tell the sound
of the small shells that are more dangerous, because they don't know how to act when
their position is overrun by the enemy.

All of this lack of
knowledge explains why so many more new recruits die when compared to the old
hands.