I assume what you write of are storyboard (conte, 
scenes mapped out on vertical boxes mostly) and 
character/prop/background design (settei, 
drawings of characters or special items, often 
from different angels and with different 
expressions) copy sketch sets.  I believe/what 
I've been told is that these are generally 
distributed to people animating/directing the 
anime such that they can try to keep their 
drawings in spec with the art style of the series 
and what the direction team has in mind for the 
scenes.  They're like guides.

I believe quite a few get made (especially 
because there will be conte for each episode and 
usually some special settei too (for characters 
of the episode at least and special outfits, 
poses, lighting or equipment the main characters 
might use, and usually the general ones as well)) 
but it seems to vary how many rise to the 
surface.  I'm also guessing with more interest in 
them, more might become available.  Of course, 
it's always difficult to know for sure if it was 
used in production or 'just a copy of that still'.

Keys
who likes the published materials collection 
artbooks additionally, but they don't have all of 
the images used of course