Explanation

Glucose molecule contains-6 carbon. For fixing one carbon ($CO_{2}$). Calvin cycle needs 3 ATP and 2 NADPH. Then for fixing six carbon $(C_{6}H_{12}O_{6})$, Calvin cycle needs 18 ATP and 12 NADPH. The net reaction of $C_{3}$ dark fixation of $CO_{2}$ is 6 $RuBP + 6\ CO_{2} + 18\ ATP + 12\ NADPH$