A quantity input field can be enabled or disabled on product pages. If you aren't seeing a quantity field it's likely it has been disabled in your theme settings.
Go to Online Store > Themes > Customize
From the dropdown in the top bar select Products > Default product
In the Product section, click into the Buy buttons block
Scroll down and enable Show quantity input
Hit Save
Start by connecting your source repository and choosing the pages you want to publish.
You can then browse your documentation and ask questions about the content your team has shared.