The WooCommerce Advanced Quantity plugin lets you control how the quantity field for each product is working.
Right now (v. 1.5.6) you can change the minimum value of the quantity field, you can set a maximum value, you can change the interval between each step in the quantity field, you can set a standard value for the quantity fields and you can add a price suffix.
This gives you a lot of possibilities to control the quantity of each product that a customer can buy. Proudly made by Morning Train.
Allow Decimal Quantity
You can allow decimal numbers as quantity (ideal for shops that sells food, textile or other products measured by weight or length/height – piece goods). For example, allow your customers to buy a tenth of a product; 0.1, 0.2, 0.3, …, or only let them buy half products: 0.5, 1, 1,5, ….
Control the Interval between each Step
You can decide an interval between each steps, allowing you to control how many products the customer has to buy at a time (ideal for shops with products packed in packages containing x products each – packed products). For example, make sure that the customer only buyes 5, 10, 15, … pieces of the product.
Set the Minimum Quantity
You can force a minimum quantity a customer has to buy (ideal for wholesale shops – bulk products). For example, set the minimum value to 10 means your customers has to buy at least 10 pieces of the product.
Set the maximum quantity
You can control how many of a product, that a customer can buy on each order (Ideal for limited offers). For example, set the maximum value to 30 means, that a customer can only buy 30 of that product in one order.
Set a Standard Value
You can set a standard value for the quantity fields. For example, setting the standard value to 7 means the quantity fields shows 7, but you can still buy less or more than 7.
Set a Price Suffix
You can set a price suffix. This is a text, that will be displayed behind the price. For example, setting the price suffix to “pr. kg.” will show that text behind the price.
Five Star Ratings
v.2.2.1 - Fixed problem their the customer could change the quantity to an invalid number on the cart page v.2.2.0 - Added compatibility with WooCommerce v.3.0.0 and up - Fixed quick edit would delete the settings for the chosen product or category v. 2.1.3 - Fixed order again issues v. 2.1.2 - Fixed some of the category options not making a different to the product - Added dropdown to input picker for the whole site - Added step intervals to categories and the whole site v. 2.1.1 - Fixed problem with PayPal for products with long names v. 2.1.0 - Fixed problem with dropdown and numbers above 1000 - Fixed problem with PayPal and products with variation v. 2.0.2 - Changed  to array() for better compatibility v. 2.0.1 - Fixed Problem with displaying max reached message on products without max property v. 2.0.0 - Added step intervals - Set quantity suffix per product, category and for the whole site - Fixed missing price suffix for the whole site - Added support for three different quantity input (Slider, (+/-) and dropdown) - Added individually variations - Fixed problem with PayPal and WooCommerce update - Removed input field and added message, when max is reached v. 1.5.8 - Implementet "pattern" and "inputmode" in quantity input template added in a lately WooCommerce update v. 1.5.7 - Fixed a bug with maximum quantity v. 1.5.6 - Fixed some bugs and incompatibility with other plugins - Added german translateion (Thanks to MEXX) - Solved a problem with price suffix on category level v. 1.5.5 - Fixed bug with add to cart link for grouped products v. 1.5.4 - Removed some debug code v. 1.5.3 - Added Quantity to add to cart URL v. 1.5.2 - Fixed a problem with stock maximum v. 1.5.1 - Fixed a problem with quantity updating in cart v. 1.5.0 - Fixed problem with validation in some browsers - Added error message for wrong quantity input - Added support for standard value and minimum quantity for the “Add to cart” button on product archive pages - Added price suffix support - Added PL translation (Thank you to Themaniac for the translation) v. 1.4.1 - Fixed problem with WooCommerce backend and update quantity v. 1.4.0 - Update service added (automatic update checking) - Fixed problem with add to cart button with no quantity field (now adds minimum quantity) V. 1.3.2 - Fixed issue when no terms assign to a product - Fixed issues with translation, and added translation description in the documentation v. 1.3.1 - Fixed PHP bug when checking the updated quantity - Added decimal functionality to WooCommerce normal PayPal payment gateway v. 1.3.0 - Added sold_individually functionality when minimum and maximum quantity is equel v. 1.2.2 - Fixed a problem with standard value, when no maximum value was set v. 1.2.1 - Fixed a problem with variable products when no minimum quantity was set - Fixed a problem when quantity in cart was updated v. 1.2.0 - Added max quantity functionality - Added standard quantity functionality - Fixed problem with quantity field on cart update (if step value and min quantity was not equal) - Fixed WooCommerce bug with product variation - Call another filter with another priority, so it will overrule other plugins with some of the same settings - Unified some functions for easier modification v. 1.1.0 - Set minimum value per category - Set minimum value for the whole site - Set step value per category - Set step value for the whole site v. 1.0.0 - Set minimum value per product - Set step value per product