To control a single product set up with 18 different options and different amounts in stock you would use our variants feature. This feature includes the inventory for each option. Here is an example with a few options along with the layout used for variants.
SKU,Price,Ounces,Stock,Option1Value,Option2Value,Option3Value
Vanilla,12.00,60,12,Vanilla,,
Chocolate,12.00,60,10,Chocolate,,
Strawberry,12.00,60,7,Strawberry,,
In this set up, the Inventory or Stock as we label it, is 12 vanilla, 10 Chocolate and 7 Strawberry