How many CO2 Sensors do I need to place in occupied large space?

In my opinion one (1) sensor is sufficient if the space is open (there are no impediments to the diffusion of the gas) and there are no local concentrated point sources of CO2 generation other than the occupants.

You don't need any CO2 sensors if it is one big space. You only need CO2 sensors in densely occupied spaces. For units serving spaces that are at least 20% not densely occupied you need to provide outside air flow monitoring to achieve this credit.

Hey Dylan! Spot on point. However I seem to remember that if you put CO2 sensors in all spaces served by a system then you can meet the credit requirement without the airflow measurement even if the spaces aren't densely occupied. I used that for a project where we couldn't accurately measure the outdoor airflow. Does that compliance path ring a bell for you?

That'd be nice. But I don't think you'd get a good enough sample size in my opinion. CO2 takes a while to mix.
I've heard of projects trying to put a CO2 sensor in the main return duct - which would be a better indication of average CO2 - but that hasn't been allowed as a replacement to the measurement by LEED in the past.

Yeah, return air is no good because if the outdoor air bypasses the breathing zone it'll give you false sense of over ventilation when the opposite is true. I'm going to personally test the diffusion speed in a large volume space we're working on. Unfortunately I won't be able to do that until late 2016 probably.