These are suitable if you order in the lowest quantity you would use to sell in and there is only one item in the recipe
You choose the inventory item which is already linked to the quantity that Product is purchases / Delivered in.
Then you choose an amount which is a multiplication of the inventory item size
Example: If you had beers which you purchase as individuals you could then set the amount in the simple item recipe to one.
If they come in a package of twenty four, you could set the Product Receiving unit to be a package of one and when you do your purchase order choose package of twenty four from the list. Although the default will be package of one. That way you can use the amount of one in the Recipe