every single carbon that goes into the cycle leaves as a CO2, if you fully oxidize 1 mol of glucose, you just created 6 mols of CO2.
my biochem prof always used to say "producing energy is just cleaving off the electrons, then what are you left with? - carbon skeletons"
as far as I know the PDC causes you to loose 1 CO2 per g3p that passes through, then your last 2 co2's are lost in the first two reduction steps of nadh, times that by the 2g3p's you got from 2 glucose and thats 6CO2s. If you're thinking of the electron transport chain as another source of CO2-- it isn't, that produces water when the electrons you cleaved finally combine with oxygen
edit: you might be confused with the overall turning of the cycle, some people think of the cycle as 1 turn per/1 glucose or 1 turn per/1 g3p, basically there isn't really a straight answer for this because the cycle is always in motion and the carbons that are lost as co2 aren't even the ones that entered in in the previous step
