If you have six hens and a rooster, and you want to harvest 120 chickens a year, that means you need to hatch 120 chicks a year. A few will go into the flock each year and a few older hens will be harvested each year to keep the flock productive.
You are going to need a couple of broody hens to hatch out 120 chicks a year.
So I would say six egg layers, two broody hens, one rooster, and you will hatch out 30 chicks every three months.
Here is what I would do: six Rhode Island Red hens as layers, two Buff Orpingtons as brooders, and a RIR rooster.
I have to consider losses during incubation and rearing processes in addition to culling of adults as productivity declines. Depending upon rearing system and its effeciency, you will always have to start out with more birds than actually needed to increase odds enough will reach usable age / size. If 120 chickens sought, then 180 would be start. Some "over-capacity" on the hens for hatching eggs also warranted when such eggs desired outside peak egg production season to ensure a more continous supply of birds for harvest.