Most people don't get more than 12 or so from fancy breeders...but some do like me, I think it just happens I got a "good" couple lines of fancies.
This is what I normally do when I am setting up new families/colonies and why I get high numbers maybe...
I get two different (well hopefully) lines of mice, one family from a store locally and another from a store about an hour away. The female mice from store A get set up with the male from store B, and vice versa. I don't know why I started doing it this way, it might not make any differance but who knows, anyways. Then I just watch.
I see which mothers are having the biggest litters, the weaker ones are snake food right away. From these best mothers, especially the ones who seem to care for everyones babies at once including their own are the ones I take offspring from. and I use this as my real colony breeders as the originals quickly burn out. I place them in with a male of the other familys good mothers. Etc. I make two or four familys like this and just take new future breeders from the best mothers. I doubt this has anything to do with large numbers but it works well for me.
Right now I have two fancies from a mother who had them in a litter of over 16. Both females had 14 pinks in their first litters. Second litters are yet to come. I am using these babies they had as breeders for feeders as they will produce like their mother and grandmother hopefully. And I use these two high producing mothers as breeders for breeders

Just selective breeding because I have yet to get IRC mice or anything of the like.
marisa