Under my "young" broody, I have eight eggs under her, and after candling, I think I'm going to get a good hatch of 5-6 chicks.
Out of twelve eggs: 6-8 eggs is average, depending on circumstances of course. Such as,
Fertility:
Good fertility rate rooster to hens:
Small rooster: 1:18
Medium/Average rooster: 1:12
Large rooster: 1:8
And age of broody:
As a hen gets older, the hatch rate goes down. Such as, a young hen will hatch 6-8 eggs out of 12, whereas an older hen might hatch 3-5 eggs out of 12.
I have in some cases have had a 100% hatch rate before. This was under my bantams. I placed 3 Standard-sized eggs under a bantam once, and she hatched all three. On another bantam, I put 5
TSC Silkie eggs under her, and she hatched out all five.
It can really vary from bird to bird, breed to breed. But the biggest things are 1) age of broody, and 2) hens to rooster: fertility.
~Lacy Duckwing