In case your supplier has a minimum purchase quantity you can order, you can easily set up that quantity in your product.
Go to your list of products and services.
Inventory > products and services
Open wanted product and you will see the minimum purchase quantity field. In this field, just type in the minimum quantity you need to order. And save the changes.
Now, when you generate a purchase order for that item from your supplier, the system will automatically populate the quantity field.
The minimum purchase quantity can also be defined in your supplier's price list.