It varies by breeding pen how many I hatch out. If I am working with a new breeding match--trying a new cross, or if I know the percentage correct color ect. will be lower because I am working with recessive traits--then I hatch accordingly. With something 'new' I really like to look at at least 40 progeny to keep 2-3 birds
If I had a good pen the previous year with established lines and am repeating the cross, then 20 is usually enough to keep 4-5.
I am finding as I work with more and more lines of chickens that most of the priciples of breeding horses and rabbits also apply. In general I observe that most breeders are 'Rule of Ten' breeding programs (aka hybrid breeding programs: raise ten to get one good bird). There's nothing wrong with that, it just takes more room and $$ to get what you want. I have run into a few 1:2:1 breeders--those running a good true line-breeding population. I always strive for 1:2:1 (one progeny exceeds parents:2 equal parents:1 is lesser than parents)--not there yet with my birds. I've got 1-2 more generations before I get the COI up to where the progeny starts getting really predictable.
I raised 50 LF Cornish this year, from 2 crosses 30 from 1 and 20 from the second. I will do a medium-hard cull at 6 months and those that don't make the cut will go into the freezer. I've worked with these birds the longest--there should be 10 keepers of that 5 will probably make it back into the breeding program.
I raised 35 LF Doms--I am culling down to hopefully a trio, the rest will go in the freezer.
I raised 70 MGB from three crosses, I will keep maybe 10. Those with DQs or serious color/conforamation faults will be removed, the rest will be sold.
So hopefully I will go into winter with:
4-6 LF Cornish '09 breeding birds
5-6 LF Cornish '10 Hatch
3 LF '10 hatch Doms
6 MGB '09 breeding birds
6-10 MGB '10 Hatch
6 mixed breed LF Laying hens (just for pretty eggs)
Then I am looking to buy 4-6 more birds from breeders, if I find what I need. I've been looking at another bantam breed, but it's hard to really think about it when I'm in the middle of culling season.