Connectr can be used to keep quantity in sync between Amazon and Shopify. Inventory can be updated with either of the 2 options:

Option 1: As available on Amazon 

Connectr will update the inventory quantities as per values from Amazon.

Option 2: Fixed count 

Connectr will set the inventory quantity on Shopify to 1, regardless of your quantity at Amazon. When you sell the product on Shopify, instead of delisting it, the quantity will set to 0 so that you don't lose your SEO on the listing.  Later when you sync the products, the app will reset the quantity back to 1 only if that SKU is in stock on Amazon.  This way, Connectr keeps your quantity in sync and ensures you never double-sell an item. Also, you can change the 'Fixed Count Quantity' field to set more than 1. This will update products with a quantity of this value. If you change this setting, all your existing products will be updated automatically when the app syncs.