how to set minimum quantity of products in the shopping cart?
You could try using the Commerce Minimum Order Quantity and Increment sandbox module. 


  The module allow you set minimum order quantity and an order quantity
  increment. This quantity is for the entire order not each product line
  item. Your shoppers will not be able to check out until their order
  quantiy reaches the minimum and/or meets the increment.
  
  Download this module and install it. Go to admin/commerce/config/moqi
  to config.


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