How many plants to justify CO2 enrichment?

Well? Is there enough CO2 in the air that 2-3 plants don't need the extra from enrichment?

Or is it helpful/beneficial regardless of number of plants?

TIA

helps regardless,.. in veg and buds, if its in the budget go ahead and do it

Doesn't matter how many plants, it's more about what kind of efficiency/productivity you're looking for. You won't need CO2 if you have proper ventilation and circulation. However, CO2 supplement boosts plant growth and allows for a completely closed system. No escaping odors means more stealthiness.