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.