You will be able to add and edit banner images from the theme customizer.
Once you are in your Shopify dashboard, go to the online store, here under the theme list you will be able to see a “Customize” button. This button will open the theme customizer.
On the theme customizer, on the left side of the screen select the section called “Slideshow”. Here you see various options to add or edit the banners.
And yes the theme supports multiple banners.