If you have more reactors and you can stagger them, then you can get more consistent CO2 levels. A single reactor peaks and then gradually falls off over the course of 2-4 weeks, giving inconsistent CO2. If you change one reactor one week, and the next the week after, than you will always have one reactor at full strength and one declining, giving you more consistent CO2. That being said, the more reactors you have, the more difficult the setup becomes (and therefore the more things that can go wrong), so it becomes a balancing act.