I don't think there is a legal limit?

You could have as many as you want.
The problem would be if you wanted to call them 'free-range' which I think requires them to have 4 square metres for each bird. I have no idea what that translates to in terms of hectares and also no idea about ducks.
If you have more than 50 then you must register with DEFRA.
What some people like to do is split their land up so that one part is getting rested while another part is getting trashed by the poultry
