In the photosynthesis reaction, the overall equation is commonly represented as:
\[ 6 , \text{CO}_2 + 6 , \text{H}_2\text{O} \rightarrow \text{C}6\text{H}{12}\text{O}_6 + 6 , \text{O}_2 \]
In this reaction, 6 molecules of carbon dioxide (CO₂) are used to produce one molecule of glucose (C₆H₁₂O₆), which contains 6 carbon atoms. Therefore, the answer regarding how many carbon atoms combine in the photosynthesis reaction is:
6