Grow room question

Ok my question is I have a grow room 20 by 15 ft roughly and I have 21 plants all a different stages my question is what size exhaust fan do I need I currently have two small fans I got with my two tents as a kit one pumping air in from outside and one pumping out but I don’t think they’re big enough and then there’s c02 which I haven’t a clue about

You need to find volume of room. Length x width x height. Then get fan(s) sized to exchange entire room once per minute. You’ll need to account for static load of duct, filters, and intake.

So if your room is 20x15x10, that’s 3000 cubic feet. Derate accordingly from there to account for whatever you have. Let’s say you have 50% reduction for example, then you would need 6000 cfm worth of fans to get one air exchange per minute.