The screen paging indicator only appears when manually browsing through your prototype either using the left/right arrow keys or the left/right arrow buttons. If interacting with your prototype via hotspots, those numbers won't appear.
To prevent those screen numbers from appearing on your prototype:
Go to your prototype, and in the top right corner, click Share.
At the bottom of the share modal, Click Link Settings.
Toggle the switch OFF next to Allow access to all screens. This ensures that the viewer will only be able to navigate your prototype via hotspots, instead of paging through it using their arrow keys.
If you have Allow access to all screens toggled ON, viewers will be able to navigate through your prototype using their arrow keys, and they’ll see your screen numbers when doing so.
If you’ve previously created share URLs that do not have this setting applied, you’ll need to revoke those if you no longer want them to be accessed.