In the photosynthesis reaction, the overall process can be summarized by the equation:
\[ 6 , \text{CO}_2 + 6 , \text{H}_2\text{O} \rightarrow \text{C}6\text{H}{12}\text{O}_6 + 6 , \text{O}_2 \]
In this equation, 6 carbon dioxide (CO₂) molecules are utilized, which means that a total of 6 carbon atoms are combined in the reaction to form glucose (C₆H₁₂O₆). Therefore, the correct answer is:
6