When you add a new product, you can choose its unit (pcs, kg, m, cm, etc.) and set the minimum and maximum purchase quantity.
How to change product's unit
Go to the Products section.
Open the product.
Select the unit from the Unit menu in the Purchase section.
By default, the unit is pcs, but you can change it, e.g., to kilograms or meters. You can also change the default unit in your merchant account's localization settings by clicking Account at the top left corner and then Settings > Localization from the left menu. If most of your products are sold in another unit than pcs, it might be a good idea to change the default as well.
How to set minimum and maximum quantities
The Purchase section also allows you to set the Minimum and Maximum quantities. These fields limit the number of products that customers can buy in one order.