Each product variant has an inventory property, which is an object with details about the total quantity and individual quantities of a variant across a merchant's locations. If no locations are available for a merchant, the locations property will be null.

{
  // ... Product Variant fields
  "inventory": {
    "total_count": 30,
    "locations": [
        {
          "id": "39072856",
          "name": "CA Warehouse",
          "address1": "544 Guerrero Street",
          "address2": "Apt 3",
          "city": "San Francisco",
          "postal_code": "94110",
          "region": "CA",
          "country": "USA",
          "updated_at": "2016-06-23T09:09:34.752Z",
          "available": 30
        }
      ]
  }
}