Right now I just do one at a time, as I need (I don't plan on freezing them), however I built it so that the whole black assembly (Made of rubber) is easily detached and a larger container can be put on. As for measuring, I wasn't aware of a way to measure co2 levels, I just assumed that as long as the mouse wasn't 'gasping' that the level wasn't too high. However, there is a control valve on the remote line itself.