I was really surprised to find that Stock Management was not included in Drupal Commerce KickStart 2 as it is generally a standard feature in any shopping cart platform.
Luckily, without much effort we can add Stock management using a contrib module: https://www.drupal.org/project/commerce_stock
Installation and setup
1. Download commerce_stock
2. Enable the Commerce Stock API & Commerce Simple stock modules
3. Make sure you enable the following: Commerce simple stock, Commerce simple stock rules, Commerce stock API and Commerce stock UI.
4. Go to Menu » Store settings » Stock management (admin path: /admin/commerce/config/stock)
5. Select the “simple stock management” tab
6. Check the product types you want simple stock to manage and hit submit
There was one thing missing and that was stock value does not appear on quick edit form. This can be added by installing commerce_stock_quickedit module: https://github.com/lanette/commerce_stock_quick_edit/archive/master.zip
Note: Quick edit form is part of Commerce BackOffice if you are not using Drupal Commerce Kickstart 2 distribution.