In this paper, we introduce a mathematical model for estimating the use of defect inspection capacity. Until recently, the selection of lots to be inspected was only done at the beginning of the manufacturing process. With the introduction of dynamic controls on production tools, the selection of lots to be inspected is done according to the production state. Our problem focuses on the Wafer at Risk (W@R) on process tools. The W@R is the number of processed wafers between two control operations. The W@R depends on several factors such as the availability of measurable products, control limits, defect inspection capacity and defect inspection control plans of products. Our model aims at calculating the defect inspection capacity required for given values of the listed factors. Experimental results on actual factory data are presented and discussed.