The quantity won't be available until when the inventory is updated. May be you can queue your inventory updates automatically, but I'm not sure that's something Shopify offers.
I don't think of any app that has the function you mention, especially showing estimated quantity (you can add code to show current product quantity only). Unless you know how to code and add custom code to each of your products. Or using Custom Product Page Builder like PageFly and add text manually to each of your product with drag and drop, no coding needed.
There is an alternative Stock Alert that can notify your users when it's restocked by email marketing, if you think that's okay then you can try out these 2 apps: https://apps.shopify.com/customer-back-in-stock-alert-user-notification-app or https://apps.shopify.com/watchlist
Hope you find this helpful
Hi Tracy @TracyH2020 ,
I sent you a private message couple weeks ago but in case you missed it I wanted to follow up from here as well.
We are working on building features around the incoming inventory problems. I would like to invite you as a beta user and get your feedback on the new features on our app.
Please check your inbox for my contact details.