As far as I know, they look at the total number of chooks - my friend's very free and easy ranging bantams increased massively one year ( the foxes must have had a set back...), and the "official" lady said - oh, you have more than 50, this is a commercial flock - to which my friend replied - we still have to buy eggs for our family, as we never find more than one or two (this being the reason why there were so many chickens in the first place!). So rather than have the hens registered, she opted for giving away and in the end culling a lot of them. They don't "keep" hens, the hens just live there.
(Sod's law, next year the foxes recovered and she ended up with only 3 or 4 hens herself, plus one cockerel...)