How much anti freeze do I put in my 2007 chevy cobalt ls.I had a new thermostat put in and what kind of anti freeze do I use

A 50~50 mix from the auto parts store will do just fine but have you considered doing a coolant system flush? Now is a perfect time.

Be sure the antifreeze you buy is labelled Dexcool or dexcool compatible. That is what GM put in the system originally and it is not compatible with non dexcool. Also it is quite difficult to fill modern cooling systems without ending up with air trapped in the system. You may need to get it done by a shop with vacuum equipment that eliminates the air pockets.