2 atoms
Just one. A molecule is the smallest unit of a substance, so by just having "CO2" implies "1CO2". A coefficient is needed to indicate more than one molecule, such as "6CO2" which is six molecules of...
One carbon and 2 oxygens = 3
If you are referring to the combustion of glucose (of which respiration is a variant) then there are 6 CO2 molecules released per glucose molecule.
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