It would be great to have a possibility to sell products in parts. At the moment you need to use modules for this like the following ones offered by customers:
Sell your products by meter, kilogram and liter Module https://addons.prestashop.com/en/sizes-units/46624-sell-your-products-by-meter-weight-pounds-liter-.html
Sell your products in fractions by Product properties addon https://store.psandmore.com/prestashop-17/62-product-properties-extension-324.html
*in comments you can write what module you prefer or suggest the one you use