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.