We have quantities set up for each variant and we set the quantity of the main product to the total of all the variants for a product. While the quantity of the variant changes when an item is purchased, the quantity of the main product does not change.
So for example, if we have 5 variants with 2 items available for each we have 10 products. If we enter 10 into the main product quantity and then an item is purchased, the variant will show a change from 2 to 1, but the main product quantity remains 10 and does not change.