If your Amazon listing is showing 0 quantity after going live, it means that your listing under Amazon’s FBA program. All you simply have to do is to make sure you convert the listing from “Fulfilled by Amazon” to “Fulfilled by Self”.
You can simply do this by clicking on the edit button of your listing in the inventory page.
Once you are done, your listing will be editable and you will be able to make changes to the quantity of your amazon listing.
Set Amazon Fulfilled by Self during “Add a Product”
While creating your listing on Amazon, you should also be able to directly set your listing to be fulfilled by Self rather than by amazon if FBA is not the route you would like to go.
Simply go to offer details in the create a listing page and you should be able to select Fulfilment Channel to fulfilled by self. 
Once your listing is approved and gone live, you will be able to edit the quantity as per your liking. Do note that for Amazon’s Fulfilled by self program, there is no attached courier to send or deliver your items, you will need to process your own orders by delivering your orders directly to your customers.