It varies by plant type, health, and other factors. So it's hard to be specific.
https://www.sciencefocus.com/science/how-many-plants-would-i...
>back-of-an-envelope calculations suggest you’d need around 400 houseplant-sized plants.
https://www.sciencefocus.com/science/how-many-plants-would-i...
>back-of-an-envelope calculations suggest you’d need around 400 houseplant-sized plants.
I have 18L of Spirulina, which appears to be having some effect, though not anywhere near a complete offset. Though, my setup is quite sub-optimal, and I've seen articles that suggest Chlorella is a couple times more efficient at biofixation than Spirulina. Continuing to tinker :)