In the material master (MRP 1 view) for the material, you have set the indicator for WI for Least unit cost procedure in the Lot size field and the Lot size independent costs and storage costs indicator.

Starting from the shortage date, successive requirements are grouped together to form lots until total costs per unit reach a minimum level. The total costs are equal to the sum of the lot size independent costs plus the total storage costs.

The most appropriate lot size is 2000 pieces.