If you set up the number of items in the pack as an option, you can set the quantity multiplier for each value to set the number, which will be deducted from the main product stock for each option value. E.g., if your customers choose the pack of 6 items, 6 items will be deducted from the stock. If your customers choose the pack of 9 items, 9 items will be deducted from the stock, etc.