The essence is that each frequency alone is eventually saturated. (can't absorb more than everything). If you add more CO2 you'll start to absorb more in new frequencies, but the effect is getting smaller. To calculate how much the numbers add up, there is HITRAN-Database with a lot of data about the absorption lines.
About these calculations there's interesting material out there. I think there's even software to download, and feed with a download of HITRAN-Data.