The above post does give excellent advice. I have flirted with the idea for years, raise a large garden, we have our own cattle, and I have the chickens. It is more difficult than one might think to raise all of your own food. There are a lot of things that can go wrong. And really, you can loose a lot of money if you are unlucky. For me, and granted, I rather play at this idea, but some years the chickens do well, some years the garden does well, some years you get great potatoes, and some years you don't.
If you don't have a lot of experience with chickens, you might consider starting with a dual purpose breed, like the Delawares. Big meaty birds, but do give you eggs too. They are slower growers, so do not all have to be processed all at once.
The true meat birds are on a FAST growing schedule, and there comes a point that after that age, their heart can't keep up. If you don't get them butchered on time, you can loose a lot of them.
As to number, start out smaller, and gradually go bigger. There are 52 weeks in the year, do you want one or two chickens a week? Would it be better to butcher a couple or a dozen at a time? How much help can you muster each time? And do you have freezer space?
Mrs K