How many chickens can fit in a 8x6 shed
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It really depends on whether you will be free ranging them or not - if so, they would only use the coop for laying and roosting and thus it could accommodate more than if you are not free ranging them.
The rule of thumb is 4 square feet of floor space per bird for the coop and 10 square feet per bird for the run. Make sure that you're not including space occupied by nest boxes, feeder and waterers in your total space. More space is always better than less space.
This thread should help, but there are many others on the topic