Sell by the case and adjust stock accordingly

I have an item that is sold individually, and by case of 8. I currently have it as a variable product with one variation for a single product, and one variation for a case. The problem is that since they are both the same product, I need to calculate the stock, but when someone buys a case of 8, it won’t deduct the 8 units from the stock, but rather only 1 unit. Is there any way to have one product and keep track of the stock? Thanks!