I can get a dozen fertile eggs but is that too many under my Buff Orpington, especially if it's a fall hatch? I'm afraid she won't be able to take care of that many. What happens when the weather gets cold?
My cochin(s) went broody at the same time, so, i put about eight under each, not they have ended up with ONE EACH! First time though, so I recomend around 8 if its her first time, so she doesnt crush them,
I had to take some chicks away from my evil (terrible mother, mean) BR and passed them off onto a RIR who was broody for the first time- between those and hers, she raised 14 just fine, but she was the alpha hen, so no-one messed with her ever. That probably helped. She had been sitting on 19 eggs before I found her secret nest and reduced, so I don't think a dozen is too many for her heat-wise either.
It depends a little on the hen, but I'd put them all under, especially if they're shipped fertile eggs. Just be sure to check for duds and pull them out after a little while.