I want to set minimum quantity of goods that customer may order in the shop. For example minimum quantity is 20, so if customer order less then 20 his order won't process and there will be message telling him that he must purchase 20 or more and redirecting him to his 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.