As it says 'agreed number of poultry' in the original contract
Sadly, I'm fairly sure that they could argue that zero is the only number they'd agree to.
On the other hand, telling you in writing that they've given you the wrong contract was not a good move.
Good luck.
I think you'll find that zero is not a number just like you can't have a minus number. "We will allow you to keep minus 6 chickens" ... Oh yes you can keep chickens we will allow up to zero. Absolute nonsense :!:
Compensation a the very least :!: