To add information about a new lot, you can go (from the "gears" menu) to "Inventory Settings" --> "Inventory" and then search for the correct item ("Rabies Nobivac" for example, or whatever you've named it). Clicking on the name of the item will bring up the editor:
Clicking that tab on the right side ("Lot Management") will get you to this view:
You can click the "Add Lot Info" to add a new lot, and you can click the "X" above "Lot Number" to remove any lots that you have used up. You can keep as many lots open as you need to, there's no requirement that you delete one before adding another.