The new iPad Pro 11 and iPad Pro 12.9 may support up to 16 GB of RAM, but iPadOS cannot use more than 5 GB. The latest iPadOS beta suggests that this could be about to change, though.
https://www.notebookcheck.net/Apple-increases-the-5-GB-RAM-limit-in-iPadOS-for-supported-devices.547503.0.html