1) How long do you grow Orpingtons before they are ready to butcher.
A very complicated question. What criteria is important to you? Is it just pure amount of meat? How are you going to cook them, that affects the age you butcher them. That age can be different for cockerels and pullets. The boys develop texture and flavor faster than the girls. How do you feed them? If you buy all they eat you may want to butcher them earlier than if they forage for a bunch of their food. How much freezer space do you have? If you butcher a bunch at one time can they all fit in your freezer? And all kinds of other questions. You never know what criteria becomes important to you until you experience it.
2) How many meals could you get out of a one Orpington ( I know that they are big and I hoping that I can get more than one)
3) How many do you butcher per year to provide chicken all year round, ( also, how many people do have to feed because I know that it will affect it.)
Not just how many people but how do you cook it and serve it. In serving pieces or in soup or maybe tacos? Plus how many times a week do you eat chicken? Pullets will never provide as much meat as a cockerel.
4) Obviously I only have so big a coop, and so big a yard and if I am raising 15 birds they will not all fit into my coop. What do you guys suggest?
I'll try to go through what I do. I don't raise Orpington though Orpington are part of the genetic background of my mutts. My mutts are equivalent size to a full sized chicken. There are only two of us and we eat chicken twice a week. Thursdays I bake a chicken for supper and Saturday we have chicken soup using the leftovers. If we are eating a pullet that uses up all the meat. If we are eating a cockerel I have enough leftovers for lunch once or twice a week.
We don't eat chicken every Thursday. Some Thursdays we see a play and go out to eat with friends (at least pre-covid). Sometimes we travel to see out grandkids or maybe to a wedding or funeral. Over a couple of seasons I learned that right number of chicks for me to hatch was between 40 and 45.
With my garden and orchard, freezer space can be precious. I don't want to spend the money for another freezer so I manage it. I typically hatch out about 20 chicks in February in my incubator so I don't run out chicken in the freezer before July. After that I may hatch another smaller batch in the incubator or can use broody hens to hatch and raise the rest. It just depends on how it goes. My overwinter laying/breeding flock is down to around one rooster and 6 to 8 hens but in summer I may have over 50 chickens down there at any one time, all various ages.
I have an 8' x 12' coop, a 4x8 grow-out coop, and a 4x8 shelter that I can house them in. I have a 12' x 32' main run (divided) and an additional 3,000 square feet inside electric netting for them. Where they sleep and what areas they get to use during the day depends on circumstances. To me it sounds like you will need to build more facilities.
Mine get to forage in that 3,000 square feet but they also get kitchen wastes and garden wastes and excess. I don't buy all that they eat so I don't feel a strong need to save money on feed by butchering them at a young age. My preference is to butcher the cockerels by around 23 weeks but if conditions require I usually start butchering a few at 16 weeks. I usually butcher 5 to 7 at a time so by the time they are 23 weeks I can finish them off.
I save a few replacement pullets every year so I don't butcher them until I can see what eggs they are laying unless I know for sure I won't be keeping a certain one. Then I eat her earlier. That means I usually butcher pullets around 8 months of age. I rotate out some of the older hens every year to make way for the replacement pullets. Since one of my goals is to play with genetics I usually eat the rooster and keep a replacement cockerel each year. When I butcher I cut mine into parts instead of keeping the carcass whole. This takes less freezer space and suites the way I cook them.
For my cockerels and pullets I usually bake them. Thursday afternoon I coat the parts in home grown herbs and bake them at 250 degrees F for 2-1/2 to 3 hours. How long depends on how big they are.
When I butcher I save various parts of the carcass for broth. I also save the bones from when we eat them for broth. The older hens and the rooster (certainly older than 10 months) typically go into broth also. When the broth is finished I pick the meat which is good for tacos, salad, or soups. My freezer space management often includes using the frozen bones and carcass parts to make a batch of broth and pressure canning it.
Also, would I need to worry about keeping them apart from my layer?
I generally don't. My brooder is in the coop so the chicks are raised with the flock. I usually integrate them at 5 weeks of age. My broody hens raise their chicks with the flock so they are integrated when the broody hens wean them. Some years my cockerels get rowdy enough that I lock them in the grow-out coop and run to keep them away from the rest, especially the pullets, but most years that's not necessary. Maybe once every three or four years.
All of mine eat a low calcium feed with oyster shells on the side for those laying eggs. Some people like to feed their birds that will be eaten a higher protein diet. For that you may have to separate them.
There are all kinds of different ways you can go about this. It's not a case of one size fits all, you will probably go through some trial and error to find the right fit for you.
Good luck!