How many molecules of carbon dioxide are used up by the plant in producing each molecule of oxygen?

At least six

this is because photosynthesis uses Carbon Dioxide, Water and Sunlight, to produce glucose. A glucose molecule is made of 6 carbon, 6 oxygen, and 12 hydrogen atoms. Since carbon dioxide is the only molecule in photosynthesis containing carbon atoms, and it only contains one per molecule, you would need a minimum of six molecules of it combined with six molecules of water in order to produce glucose. (with an extra 12 oxygen atoms)