To create product volume discounts, click on "Create product volume discounts tiers" from the home screen of Multiscount: 

or go to the Shopify admin discount page and click on "Create discount" and select "Product volume discount":

Then select if you want your discount to be an "automatic discount" (the discount is applied automatically) or a  "code discount" (the customer needs to insert the discount manually):

Configure the tiers and criteria of your discount by setting a minimum quantity of products in the cart or a minimum subtotal for each tier. You can also set a maximum value for each tier:

Finally, set the maximum number of times the discount can be used, determine if it can be used in conjunction with other discounts, and specify the period during which the discount will be available.