It depends, how much ventilation do you have? If air is moving quickly through the space then C02 wont have an effect, if not, then I would say you would need one bottle per plant in an area that big to really have an effect. Honestly I dont think its worth the trouble, you are gonna put alot of work into something that isnt gonna help a whole lot. If you wanna
decrease yield I would concentrate on lighting and good ventilation. But thats just my opinion.
cheers.