Define Product inventory mapping
Actually, in Magento the “Inventory“ is a separate object related to the Product object. For this reason, the “Qty“ is a property of the Inventory object, to of the Product object and you should keep it in mind when create a mapping.
Instructions
Go to the “Product mapping” page
Click the “Add New Mapping“ button
Choose the “Product Inventory: Qty“ item in the “Magento Attribute“ dropdown
Choose necessary Salesforce Attribute and sync settings
Save changes
PS: