In the process of photosynthesis, the general equation can be represented as:
\[ 6 , \text{CO}_2 + 6 , \text{H}2\text{O} \rightarrow C_6H{12}O_6 + 6 , \text{O}_2 \]
This shows that 6 carbon dioxide (CO₂) molecules are used to produce one molecule of glucose (C₆H₁₂O₆), which contains 6 carbon atoms.
So the answer is 6 carbon atoms.