Hi Francis,
Simplest way is to set up an item for the crate(s)
Then whenever you sell milk, you add 1 pcs of crate for each 18 pcs of milk.
That way, you can better keep track of your crates. You will know how many you have, you will know what they are costing you. If a crate is lost, or stolen, you can handle that.
If your sales of milk increase all of a sudden, you can easily increase your stock of crates, by purchasing new ones.
if a crate is broken you can book into a "repair" warehouse, and book it back into stock.
You could even create a production item (BOM) in which you combine 18 pcs of milk with the 1 pcs of crate.
This way you can easily assign different prices for when someone buys an entire crate and when they buy less than 18 pcs of milk.
In the end you will know exactly how many you have.
Regards,
Johan