For Kareshi Kanojyo no Jijyo (His & Her 
Circumstances), the average 30-minute episode 
utilized about 2,000-2,500  cels.  This is 
according to the recap episodes I saw (~ Act 
14).  However, according to the same source, some 
episodes used as little as 600  cels while others 
used as many as 3,500  cels.

And for those of you who viewed this anime, you 
would realize that KareKano tends to use many 
still and pan shots as well as simpler animation 
compared to other anime.  Because of this, I 
would say that the numbers I mentioned above are 
not typical numbers and probably would under-
estimate the number of cels used in a typical 
half-hour anime episode.

(Please also keep in mind most cels in an episode 
are of minor character images or supplementary 
scene images--most folks would consider 
them "junk" cels or background elements.  The 
kind of cels with nice enough images that cel 
collectors usually aim for probably consist a 
pretty small fraction of the total cels used in 
an anime episode.)

Is this information and conjecture good enough 
for you?