How many molecules are there in CO2?

2 atoms

One carbon and 2 oxygens = 3

CO2's molecular weight is 44 so 44 g of CO2 contain Avogadro's number of CO2 molecules. 20 g would contain 20/44 x Avogadro's number.

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.

180