To me it seems like iOS 12 is consuming up to 7 GB of the disk on a relatively clean install.
Below are my details (same iPad model as original poster with iOS 12.4.8):
After doing a factory reset and importing a very modest iCloud backup:
After doing a factory reset and not doing any iCloud backup:
A few days after the factory reset while doing practically nothing on the iPad:
I also have an iPod with iOS 12 (not a fresh install), this is what it reports:
To me that looks roughly consistent with the numbers for the iPad.
Finally, this is what Apple states for iOS 14:
Available space is less and varies due to many factors. A standard configuration uses approximately 10GB to 13GB of space (including iPadOS and preinstalled apps) depending on the model and settings. Preinstalled apps use about 4GB, and you can delete these apps and restore them. Storage capacity subject to change based on software version and may vary by device.