In the photosynthesis reaction, the overall equation is:
\[ 6 , \text{CO}_2 + 6 , \text{H}2\text{O} \rightarrow C_6H{12}O_6 + 6 , \text{O}_2 \]
This means that 6 molecules of carbon dioxide (CO₂) are used to produce 1 molecule of glucose (C₆H₁₂O₆), which contains 6 carbon atoms.
Therefore, the correct response regarding how many carbon atoms combine in the photosynthesis reaction is 6.