In carbon dioxide (co2), there are two oxygen atoms for each carbon atom. each oxygen atom forms a double bond with carbon, so the molecule is formed by two double bonds.
two double bonds means that the total number of electrons being shared in the molecule is
two.

Explanation:

Carbon has four electrons in its outer most orbit and it can share four more electrons from other atom to complete its valency while oxygen has 6 electrons in its outermost orbit and it can either charge the electron or gain two electrons from other atom.

Therefore in , the carbon atom shares its two electrons with two electrons of each oxygen atom and forms double bond with it. Thus, total pairs of electron to be shared by the atom of the molecule is two.