Hi, it really depends on how you are running them, for instance do you require to keep them in if you are away for a weekend ? it also depends on the type of birds you are getting (i.e bantam or LF and breed). I imagine you are putting perches in, a rough rule of thumb is about 30cm of perch per avg LF Hen. If you ran that along the 4m wall and could manage to put two in whilst leaving floor space for feeders etc that would give you perch room for 26 / 27.
Also you don't say how high it is / ventilation as the more you have in the more ammonia from poo and therefore more frequent cleaning (higher roof's / sufficient vents alleviate the fresh air problems a little)
Best to buy in stages then you can see how your choice of birds fit it ( also allows laying hens to be at different stages and not all moulting together). Too many chickens means plenty of shovelling, more disease prone and more bullying / feather pecking etc as they become more competitive for resources.
Probably more important is the area they have access to outside, if it's a whole garden great, if its a small pen the 27 is a mud bath