In the photosynthesis reaction, specifically in the overall equation:
\[ 6 , \text{CO}_2 + 6 , \text{H}2\text{O} \rightarrow C_6H{12}O_6 + 6 , \text{O}_2 \]
six carbon atoms from carbon dioxide (\( \text{CO}2 \)) combine to form one glucose molecule (\( C_6H{12}O_6 \)).
So the correct response is 6.