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It is a matter of personal conscience.
If you want to push your luck and see how much you can get away with without overt catastrophes like cannibalism, then go for it. Factory-farmed chickens live with like 1-2 sq ft per chicken; you need at least 8" of linear roost space per chicken; and it usually works well to have 1 nestbox per 3-4 chickens.
If you wnat the chickens to have really good quality of life, you can't really *build* a tractor for 10 chickens unless you have a (real!) tractor to PULL it with or unless the tractor is just a nighttime roosting-box and they're free ranging all day every day.
Or wherever in between you feel okay with.
Good luck, have fun,
Pat