How many moles of Co2 will be produced if 9.0 moles of O2 completely react?

It depends on what was being combusted.

Supposing the simplest case, where carbon is being burnt:

C + O2 → CO2

(9.0 mol O2) x (1 mol CO2 / 1 mol O2) = 9.0 mol CO2

The answer would be different if the fuel contained any oxygen atoms, for example, alcohol.