The How Many? Extension works on offer-listing pages as well as limited functionality on the main product detail page.
An example of an offer-listing page is : https://www.amazon.com/gp/offer-listing/B00DYEJCJO/
An example of a product detail page is: https://www.amazon.com/Travel-Bottles-colors-cosmetic-products/dp/B00DYEJCJO
The product detail page gives you the buy box quantity as well as if other sellers are listed on the main page it will give you those quantities as well. This will load automatically so there is no need to click the extension icon.
If a product has variations like a shoe has multiple sizes and colors. A button will be displayed with the text "#? How Many Variations".
If you click on that, it will open a new tab and start populating information there about all the variations for that product.
The pop over that you see in the video is for unlimited users with all the various information is only available on the offer-listings page.