You can cancel the dynamic placement of items either by right-clicking in the Design Window or by hitting the Esc key.
The items placed in the drawing will appear in the active drawing view in the EDE and rest of the items can be seen in the Active Drawing Stockpile or in the Plant Stockpile. The system automatically tracks the number of items already placed. When you start placing the items the next time, it shows the ordinal of the item being currently placed and the number of remaining items for placement in the respective stockpiles.
To resume the canceled operation, select the item again in the EDE.