Trevor the Guru here!
If you go to your products section and click into one of your products, you'll see a "Varients" area. On the right-hand side of each variant, you'll notice an "Edit" button. If you select this option you'll be able to edit each variant with the respective quantity level.
Additionally, I was able to dig up some guides that I think would be super helpful for this situation!
Feel free to reach out to our 24/7 Support via phones/chats if you have any other questions!
All the best,
My bad, actually I want that to be for customers when buying from store (not for me to adjust the invetory)
Instead of adding one by one in the cart I want them to be able to select more than one variant and add them all in the shopping cart.
I hope that make it more clear.
This thread is quite old, but if this helps others who are looking for the same solution for showing quantity box for all the variants and allow add to cart at once.
You can use MultiVariants ‑ Bulk Order to allows your customers to order multiple variants and quantities of the same product in just one click.
Here is the demo, you can take a look.