Well any amount of co2 is good at sea level it is 300ppm in the air but i live at like 1,800 elevation plus the plant is in a box in a closet so i doesn't get much CO2 going in and out so any extra will help.

Conservatively id would say that 1-2 2L bottles will raise co2 by at least 10-30% depending on how good the yeast grows which could mean life or death for sick plants